The security headquarters in the Egyptian city of Port Said has been set alight in a second day of clashes between police and protesters. The fire started on the ground floor of the building as protesters gathered in the streets nearby. Earlier, thousands of people joined a funeral procession for three civilians killed in overnight clashes, which also left three policemen dead. At least 400 people have been injured in the fighting. Port Said has been rocked by weeks of intermittent violence. Protests, often leading to clashes, have been taking place there since January, when 21 local football fans were sentenced to death over football riots which left 74 people dead in February 2012. Violence also broke out in the capital Cairo on Monday as protesters blocked a key thoroughfare and set alight at least one police car. 'Stay away' Fighting began in Port Said early on Sunday when word spread that 39 defendants still facing trial over the riots were being moved to outside the city. Thousands of protesters clashed with police, hurling rocks and firebombs; while riot police fired back with birdshot and tear gas. The violence petered out overnight but flared again after the protesters' funerals. Angry mourners shouted anti-government slogans during the procession. Reports said emergency services were unable to reach the burning building because of the clashes. The interior ministry said that two policemen died from gunshots to the head and neck and a third died of his wounds later on Monday. Dozens of people have been injured by live shots, emergency officials in Port Said said. The army - which has stayed on the sidelines during other recent violence - moved in at one point to separate the two sides, but its troops began getting hit by live fire and becoming overwhelmed by police tear gas, Associated Press reports. The agency said protesters cheered when troops fired in the air in the direction of the police, and have been shouting: "The people and the army are one hand!", urging the military to take their side. The interior ministry, in a statement, accused unknown assailants of randomly attacking police and army personnel in the city "with the aim of creating sedition and divisions" between them. It urged residents of Port Said "to stay away from groupings near government buildings", according to AFP. 'Delay election' The latest violence comes at a time of heightened political tension in Egypt. The country has been increasingly polarised between pro-Islamists and liberal and secular forces since Islamists swept to power in parliamentary and presidential elections last year. Scores of people have died in clashes across the country this year amid protests against Islamist President Mohammed Morsi and grievances over the treatment of those convicted over the football riots. Leading opposition figure Amr Moussa on Monday again called for the postponement of parliamentary elections due in April. "A delay to the elections is the solution," he wrote on his Twitter account. There has been widespread antagonism towards the police since the mass protests which brought down former President Hosni Mubarak in February 2011. Many people believe the police stood by, allowing last February's football riots which led to the deaths of 74 people, mainly supporters of Cairo's al-Ahly team, in revenge for their support for the anti-Mubarak protests. A court is due to deliver verdicts against the remaining 39 defendants on 9 March.