US President Donald Trump has surprised supporters gathered outside the hospital where he is being treated with Covid-19 by riding past in a motorcade.
Wearing a mask, the president waved inside a car moments after tweeting that he would pay a "surprise visit".
Mr Trump, who has been criticised for his handling of the pandemic, also said he had learned a lot about the virus.
Earlier, his doctors said Mr Trump continued to improve and might be discharged as early as Monday.
Dr Sean Conley said the president's oxygen level dropped twice since his diagnosis, and he was started on a steroid called dexamethasone.
The president was given extra oxygen at least once after testing positive, said Dr Conley, who also sought to clarify earlier confusion caused by conflicting statements about Mr Trump's condition.
In a tweet, the president - dressed in a suit jacket and shirt with no tie - said: "I learned a lot about Covid. I learned it by really going to school. This is the real school. This isn't the let's read the books school. I get it and I understand it. It's a very interesting thing, I'll be letting you know about it."
The president's diagnosis, which he made public in a tweet early on Friday, has upended his election campaign. Mr Trump faces Democratic challenger Joe Biden on 3 November.
What did the doctors say?
Speaking at a news conference at the Walter Reed National Military Medical Center close to Washington DC, Dr Conley said Mr Trump's oxygen level dropped for the first time on Friday morning at the White House.
The president had a high fever, he said, and an oxygen level below 94% - a healthy person's level is 95% or higher.
The president was given supplemental oxygen "for about an hour", the doctor said, and was flown to Walter Reed in the evening. The news had already been widely reported in US media, and Dr Conley's confirmation came after he refused to answer several questions about the issue during Saturday's briefing.
The second episode happened on Saturday, when the level dropped below 93%. When questioned, Dr Conley did not say whether the president had received oxygen but added that, if it had happened, "it was very limited".
The team, Dr Conley said, decided to give Mr Trump dexamethasone, which is shown in studies to improve survival for patients in hospital with severe Covid-19.
Steroids calm down inflammation and the immune system and are already used in conditions like arthritis and asthma as well as in some severe infections. The drugs are not thought to be helpful in the early stages of a coronavirus infection.
"Given the timeline where [Mr Trump] is in the course of illness, we're trying to maximise everything that we can do for him… We decided that in this case the potential benefits early on in the course probably outweighed any risks at this time," Dr Conley said.
Dr Conley also addressed a conflicting account about the president's health given shortly after his briefing on Saturday by the White House chief of staff. Mark Meadows said Mr Trump's vital signs over the previous 24 hours had been "very concerning" and that the next 48 hours would be critical.
"I think his statement was misconstrued," the doctor said.
However, he acknowledged giving an overly upbeat description of Mr Trump's condition a day earlier: "I didn't want to give any information that might steer the course of illness in another direction. And in doing so, you know, it came off that we were trying to hide something, which wasn't necessarily true."
The president, being 74, a man and someone categorised as obese, is in a higher-risk category for Covid-19. On Friday he was given an experimental drug cocktail injection and started a five-day course of antiviral medication remdesivir.
Dr Brian Garibaldi, who is also part of the team treating the president, said: "He feels well, he's been up and around and our plan for today is to have him to eat and drink, be up out of bed as much as possible to be mobile."
The doctors said the president had not had a fever since Friday and that his liver and kidney functions had remained normal. But Dr Conley refused to answer questions on whether lung scans showed any damage.
This is the video that he tweeted minutes before going outside. It was in this video that he said that he had a surprise for his supporters outside the hospital.
Analysis by James Gallagher. Health and science correspondent
How significant is it that Donald Trump has been given dexamethasone? The steroid saves lives by calming the immune system, which can become dangerously overactive in Covid, but needs to be used at the right time. Give it too early and the drug could make things worse by impairing the body's ability to fight off the virus.
This is not a drug you would usually give in the "mild" stage of the disease. The Recovery Trial, which took place in the UK, showed the benefit kicked in at the point people need oxygen - which Mr Trump briefly did. The World Health Organization translated those findings to advise using the steroid in "severe and critical" cases.
Mr Trump's blood oxygen levels did dip below 94%, which is one of the National Institutes of Health criteria for "severe illness". However, those low oxygen levels were not sustained and the gap between someone needing transient oxygen support and end-stage Covid-19 is massive.
We do not know the full details of Mr Trump's condition, but it is hard to imagine you or I would be discharged from hospital while taking dexamethasone and remdesivir and after being given an experimental antibody therapy. However, we do not have the medical support at the US president's disposal.
Who else around the president has tested positive?
A number of people around the president have tested positive, including First Lady Melania Trump. Many of them attended a meeting at the White House last weekend that is being scrutinised as a possible "super-spreader event".
Other people to have tested positive around Mr Trump include close aide Hope Hicks - believed to be the first to show symptoms - campaign manager Bill Stepien and former White House counsellor Kellyanne Conway.
Nicholas Luna, the latest person reported to have tested positive, is a personal assistant or "body man" of the president and is in constant contact with Mr Trump.
What about the political situation?
The president's campaign team said on Saturday it would move forward "at full speed" until Mr Trump could return to the campaign trail. It is calling on top "surrogates", including Mr Trump's sons Donald Jr and Eric, and Vice-President Mike Pence to "carry the campaign forward" for the time being.
Meanwhile, Mr Pence is scheduled to debate Democratic vice-presidential nominee Kamala Harris on Wednesday.
Joe Biden, who has continued his campaign, did not have plans for in-person events or public appearances. He has taken down negative advertising about the president and on Saturday said the president's response to the pandemic had been "unconscionable".
In an interview to CBS' Face the Nation, House Speaker Nancy Pelosi, a Democrat, said Republicans in Congress and the president had "for a long time... been anti science" and that she hoped that Mr Trump's positive diagnosis would change his attitude towards the virus.
"I pray that in addition to his health, that the president's heart will be open to the millions of people who have been affected," she said. "I hope it will be a signal that we really have to do better in preventing the spread of this virus."
You lots don't live in America yet you have opinions. You cannot vote, You claim you know what's good for America with ideas in your head forming woke. You don't know anything. Trump is a national security risk. He has divided America to the point of chaos.
Trump hates Nigerians that even his new student visa rules is crazy, yet the same Nigerians worship this narcissist? I cant comprehend.
US President Donald Trump's drive-by for supporters has been criticised as "insanity" by a top doctor.
Mr Trump, 74, briefly left Walter Reed Medical Center in Maryland on Sunday, where he is being treated for COVID-19, so he could greet the crowds waiting outside the hospital.
He sat in an armoured SUV, wearing a mask, and waved to people as he was driven along the road to cheers and flag-waving.
Dr James Phillips, chief of disaster medicine at George Washington University and an attending physician at Walter Reed Medical Center, described the drive-by as "insanity".
He wrote on Twitter: "That presidential SUV is not only bulletproof, but hermetically sealed against chemical attack. The risk of COVID-19 transmission inside is as high as it gets outside of medical procedures. The irresponsibility is astounding. My thoughts are with the Secret Service forced to play.
"Every single person in the vehicle during that completely unnecessary presidential 'drive-by' just now has to be quarantined for 14 days. They might get sick. They may die. For political theater. Commanded by Trump to put their lives at risk for theater. This is insanity."
Jonathan Reiner, professor of medicine and surgery at George Washington University school of medicine and health services, tweeted: "By taking a joy ride outside Walter Reed the president is placing his Secret Service detail at grave risk.
"In the hospital when we go into close contact with a COVID patient we dress in full PPE: Gown, gloves, N95, eye protection, hat. This is the height of irresponsibility.
"His doctors should have said no. They need to stand up for what is right. This is wrong."
Sky News correspondent Sally Lockwood, who was there, said there had been a "mini rally growing outside the front of the hospital - he can hear them if he can't see them from his hospital suite, I'm sure - it has been non-stop all day".
She said the crowd "went wild" as Mr Trump was driven along the road before making a U-turn and continuing to wave at supporters and reporters as he came back.
"He looked like he was enjoying himself and his fans were impressed with the sight of the president," she said.
Mr Trump's surprise appearance came shortly after he released another video - his third since revealing in the early hours of Friday that he had been diagnosed with the virus.
In the video, he had promised "to pay a little surprise to some of the great patriots that we have out on the street".
He also described his treatment as a "very interesting journey", adding: "I learned a lot about COVID.
"I learned it by really going to school. This is the real school. This isn't the 'let's read the books' school. And I get it, and I understand it. And it's a very interesting thing."
It is a turnaround for a president who has spent much of the pandemic playing down the disease and even mocking those who wore masks in an effort to protect themselves and those around them.
The number of people who have died with the virus in the US is approaching 210,000 and the number of confirmed cases has passed 7.4 million.
Mr Trump's medical team has said that, if his condition improves, he could be allowed to continue his treatment at the White House from Monday.
But the doctors have also been criticised for presenting information that has been contradicted at times by Mr Trump's staff.
On Saturday Dr Sean Conley, who heads the medical team, had refused to admit that the president was given supplemental oxygen at the White House before being taken to hospital.
Minutes later, Mr Trump's chief of staff Mark Meadows had said medics were "very concerned" about the president's vital signs before he arrived at Walter Reed.
On Sunday, Dr Conley admitted Mr Trump's oxygen levels had dropped suddenly on two occasions and he had in fact been given oxygen on Friday.
The doctor said he had been "trying to reflect the upbeat attitude of the team" and "didn't want to give any information that might steer the course of illness in another direction".
House Speaker Nancy Pelosi told CBS: "We need to have trust that what they're telling us about the president's condition is real."
Mrs Pelosi said the information doctors are telling the public "has to be approved by the president", adding: "That's not very scientific".
Trump wish u quick recovery.. The hate dis man get is jst 2 much from the first day in office people protested in their numbers... Trump na strong man o... A lot of people died around the world due to Coronavirus not just Americans it's a disgrace they are playing politics with it..
Hmmm I dont know where this is going but either if its real of scripted, this is coming out bad on Trump.
Staying one place untill he is clear of the disease n good to go will be good, he needs to stay off the media for now, anything he says or do now will be used against him. People are out there that will spin all his moves now, to cost him votes.
If he comes out now to say that its serious, then people will say that he was very dumb for 200,000 dead bodies to not convince him. If he comes out to say that its nothing, then they will say that it was a hoax, n that he is disrespectful to the dead people.
Trump, The man who has his people in his heart. showing concerns even when he's on sick bed. BTW, If it's our own darling Bubu now. see what we'll be hearing:- *Buhari's PA have been infected with COVID-19 but Buhari is negative. *Buhari: I can't use face mask because Aso rock is secured. *8 new confirmed cases in Aso rock but Buhari is negative they keep deceiving us every day because of those Afongas who is licking Mr Integrity's Ass
