PORT SAID, Egypt - Egyptian protesters demanding the release of prisoners battled police in Port Said for a fourth day on Wednesday, challenging state authority in the turbulent city at the northern end of the Suez Canal.Unrest has plagued Egypt since Hosni Mubarak's overthrow two years ago, with Port Said a focus for violence since a football stadium riot in which more than 70 people died last year.Residents have staged angry protests since January over death sentences handed down to some Port Said residents in connection with the riot.Hundreds gathered again on Wednesday, throwing rocks at police, who fired teargas to drive them back from a local government building, a Reuters witness said.Army troops tried to separate police and protesters, and at one point a soldier was evacuated in an ambulance after choking on teargas, the witness said."Stamp your foot, shoot your gun, Port Said is free," protesters shouted, demanding the release of several demonstrators detained on Tuesday. "The people want to overthrow the regime."That was the signature chant of the demonstrators who ended Mubarak's 30-year rule in an upheaval that helped install an elected Islamist-led government which is still struggling to assert its authority, restore order and revive the economy.There was no immediate word on casualties in the latest clashes. Yehia al-Anifi, a health ministry official, said some 228 people had been wounded in fighting in Port Said on Tuesday, including some with live bullets and birdshot.At least six people have been killed in this week's protests in the Mediterranean city, including three policemen."Violence is rising everywhere, about 500 injured in one day without any effort for dialogue by the regime. Is there still a president and a government?" opposition politician Mohamed ElBaradei wrote on his Twitter feed.