By Yusri Mohamed and Alexander Dziadosz

PORT SAID/CAIRO, Egypt, March 6 (Reuters) - 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.

Violent demonstrations also broke out in Cairo, where protesters threw petrol bombs at police and soccer fans set fire to a security car, in another a sign of the insecurity that has plagued Egypt since Hosni Mubarak's overthrow two years ago.

Port Said has been a focus for violence since January, with people staging angry protests over death sentences handed down to some residents in connection with a football stadium riot in which more than 70 people died last year.

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.

In reaction to the violence, Islamist President Mohamed Mursi met with police officials to review the security situation, stressing "the importance of concerted efforts to achieve security and protect peaceful protesters while quickly controlling rioters", the state news agency MENA reported.

The interior minister also ordered that Port Said's security chief be replaced.

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.

In another sign of the problems authorities face, hundreds of police demanding more weapons after several police were killed in the recent unrest, staged a strike in a base outside Ismailia, roughly 70 km (45 miles) south of Port Said along the Suez Canal.

FIRE IN CAIRO

Protesters also battled security forces with stones and petrol bombs outside Cairo's Tahrir Square, the centre of the 18-day revolt that ousted Mubarak and a major route for traffic through the capital.

Scores of young men ran back and forth on streets covered with stones as they skirmished with police in the area along the River Nile near the U.S. Embassy and a luxury hotel.

In the southern Cairo neighbourhood of Giza, soccer fans demanding retribution for people killed in the Port Said stadium riot set fire to a police car, MENA said.

Many Egyptians have been alarmed by authorities' inability to contain unrest and restore security, especially ahead of parliamentary elections that were scheduled to start next month.

"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.

Mursi and his Muslim Brotherhood, who have dominated elections since Mubarak fell, have repeatedly called on Egyptians to end strikes, sit-ins and protests to restore stability and help heal the country's ailing economy.

Egypt had planned to begin elections on April 22 for its lower house of parliament, seen as an important step in securing a $4.8 billion loan from the International Monetary Fund to bolster the country's finances as it grapples with sliding foreign currency reserves and a swelling budget deficit.

But an Egyptian court threw that timetable into confusion on Wednesday when it ordered the cancellation of Mursi's decree calling the vote.

Some 60 people died during a spasm of country-wide unrest between Jan. 25 and Feb. 4, with many protesters calling for Mursi to step down and voicing anger over a broader sense of political and economic malaise. (Writing by Alexander Dziadosz; Editing by Alistair Lyon and Michael Roddy)