Park Won-soon went missing after leaving a voicemail that his daughter described as sounding like a will.

Seoul City Mayor Park Won-soon was found dead by police on Friday, South Korea's Yonhap news agency reported, after his daughter reported him missing a day earlier.

The Seoul Metropolitan Police Agency found his body in northern Seoul, near where his phone signal was last detected during a late night search, Yonhap said.

A massive search operation was under way on Thursday.

His daughter called police on Thursday afternoon and said her father left "a will-like" message before leaving their home four to five hours earlier, the The Associated Press news agency reported.

The daughter did not explain the contents of the message and said she decided to call police because she could not reach her father on the phone.

The Seoul-based SBS television network reported that one of Park's secretaries had lodged a complaint with police on Wednesday night over alleged sexual harassment, such as unwanted physical contact that began in 2017.

Both the Seoul Metropolitan Police Agency and Park's office said they couldn't confirm the reports.

About 300-400 officers and a drone were mobilised in the search, according to police.

Kim Ji-hyeong, a Seoul Metropolitan Government official, said Park had not shown up for work on Thursday for unspecified reasons and had cancelled all of his schedule, including a meeting with a presidential official at his Seoul City Hall office.

Who was Mayor Park Won-soon?
Park, a longtime civic activist and human rights lawyer, was elected Seoul mayor in 2011.

He became the city's first mayor to be voted into a third term in June last year.

A member of President Moon Jae-in's liberal Democratic Party, he has been considered a potential presidential candidate in 2022 elections.

Park has mostly maintained his activist colours as mayor, criticizing what he described as the country's growing social and economic inequalities and corrupt ties between large businesses and politicians.

Early on, Park established himself as a fierce opponent of former conservative President Park Geun-hye and openly supported the millions of people who flooded the city's streets in late 2016 and 2017, calling for her removal over a corruption scandal.

Park Geun-hye was formally removed from office in March 2017 and is currently serving a decades-long prison term on bribery and other charges.

Seoul, a city of 10 million people, has been a new centre of the coronavirus outbreak in South Korea since the country eased its rigid social distancing rules in early May.

Authorities are struggling to trace contacts amid surges in cases linked to nightclubs, church services, a huge e-commerce warehouse and door-to-door sellers in Seoul.

Park Won-soon led an aggressive anti-virus campaign, shutting down thousands of nightspots and banning rallies in major downtown streets.