Nigerian Police and Their Brutality in Onitsha

Why do police in South East behave like they are not trained .I travelled to my state recently(ANAMBRA STATE)On these faithfull evening i was driven back to Onitsha by seven PM ,when some MEN in mufti stopped me.I was really afraid thinking they were armed robbers,they asked me several questions ,it was then i realized that they are police men.But to my greatest suprise they asked me to give them the receipt of my GSM phone.when i started to ask them questions they brag about what they can do to me,but any way i didn't give them money but i got home very late .They arrest people at random just to forcefully obtain money from them,Police men will stop okada rider with passenger and search the passenger if they see handset they will demand for the receipt.The police is not doing anything in Onitsha apart from chasing and harassing the traders who goes about their businesses.AND why do some Nigerian police men don't wear their Uniform these days?are they all undercover agents? We need to know who is a police and armed robber.