The main security building in Egypt's Port Said was set ablaze on Monday as fighting which cost six lives erupted anew between police and protesters in the restive Suez Canal city, an AFP correspondent said.
The blaze, which started on the ground floor of the security headquarters, sent plumes of smoke into the air while clashes continued in the streets surrounding the building.
Rescue services could not reach the area, witnesses said, as anger mounted in the city where a campaign of civil disobedience entered its third week.
The unrest, fuelled by January death sentences handed down to football fans over deadly riots, boiled over again on Sunday night when clashes left six people -- including three policemen-- dead.
The interior ministry said in a statement that unknown assailants had randomly attacked police and army personnel in the city "with the aim of creating sedition and divisions" between them, following unconfirmed reports the army had fired on policemen.
It urged residents of Port Said "to stay away from groupings near government buildings."
Earlier thousands had taken to the streets for the funeral of the three civilians killed in the overnight clashes chanting against the police and Islamist President Mohamed Morsi.
"The interior ministry (officials) are cowards!" the angry mourners chanted and called on Morsi to "leave".
Aside from those killed, hundreds were injured in the overnight fighting after the authorities decided to move prisoners awaiting a verdict over alleged involvement in a deadly football riot last year.
Two policemen died from gunshots to the head and neck and a third died of his wounds later on Monday, the interior ministry said.
Of the 586 people injured in the confrontations, 16 had been hit with live rounds and 27 with birdshot, according to Ahmed Sultan, head of emergency services.
A security official said that during the night protesters threw petrol bombs and stones at the police station in Port Said, where a general strike entered its third week, and the police responded with tear gas.
The city has witnessed unrest since January when a court sentenced Port Said residents to death over a football stadium riot last year, triggering a series of protests and eventually a civil disobedience campaign.
Traffic in the Suez Canal, a vital waterway for global commerce, has not been disrupted, the canal authority said.
The interior ministry said it decided to move the prisoners from Port Said because it wanted to avoid unrest.
The court verdict, expected on Saturday, is for 39 defendants in a case which has already resulted in death sentences for 21 others, sparking clashes that killed at least 40 people.
Residents of Port Said and other canal cities have long complained that Cairo marginalises them.
Last year's football riot which killed 74 people, mostly supporters of a visiting Cairo team, exacerbated Port Said's isolation, residents of the city say.
Egypt has been gripped by nationwide unrest in the past few months, with protesters taking to the streets to denounce Morsi for failing to address political and economic concerns.
Opponents accuse Morsi -- elected in June last year after a turbulent period of military rule -- of failing the revolution that brought him to the presidency and of consolidating power in the hands of his Muslim Brotherhood movement.
The Nile Delta has also seen its share of unrest, with a civil disobedience campaign declared in the province of Daqahliyah. One person was killed and dozens injured in clashes over the weekend between police and protesters in provincial capital Mansura.