Clashes have erupted outside government offices in the Lebanese capital Beirut after thousands attended the funeral of security chief Wissam al-Hassan who was killed by a car bomb on Friday. A group of protesters tried to storm the HQ, after a new call for Prime Minister Najib Mikati to resign. Police fired warning shots and tear gas. Friday's attack also killed one of Mr Hassan's bodyguards and a woman nearby. Opposition figures have blamed neighbouring Syria for the attack. Many have protested against Syria and its Lebanese allies amid fears the Syrian conflict could spill over. The confrontation outside the prime minister's office lasted for a few minutes. Two former prime ministers - Saad Hariri and Fouad Siniora - intervened to urge their supporters to remain calm. Lebanon's religious communities are divided between those who support the Syrian government - including many Shias - and those mostly from the Sunni community who back the rebels. Lebanon's Shia militant group Hezbollah - a close ally of the Syrian government - condemned the bombing. Syrian Information Minister Omran al-Zoubi called it a "cowardly, terrorist act". He said such incidents were "unjustifiable wherever they occur". Syrian troops withdrew from Lebanon in 2005 after a 29-year-long presence, in the wake of the assassination of former Prime Minister Rafik Hariri. Mr Hassan, 47, was close to the 14 March opposition and the Hariri family, part of the anti-Syrian opposition. Blaming neighbour President Michel Suleiman and Prime Minister Mikati - who has faced calls for his resignation over the killing - greeted Mr Hassan's coffin at an earlier ceremony at the headquarters of the Internal Security Forces (ISF). Mr Hassan headed up the intelligence branch of the ISF. The car bomb exploded close to its offices in the east Beirut neighbourhood of Ashrafiya. Mr Hassan was to be buried alongside former Prime Minister Hariri. He led an investigation into Mr Hariri's assassination, which implicated Damascus. Mr Hassan also recently organised the arrest of a former minister accused of planning a Syrian-sponsored bombing campaign in Lebanon. A day after the bombing, Prime Minister Mikati suggested the attack had been connected with that case. Many mourners at Sunday's funeral waved the light blue flag of the Sunni-based opposition Future Party, while others carried Lebanon's national flag. Many people described Mr Hassan as a martyr who was killed trying to protect his country. "We came for Lebanon's future to show that we will not be scared," said one of the mourners. The prime minister offered to stand down as prime minister on Saturday, but President Suleiman asked him to stay on in the national interest. A prominent Lebanese opposition MP, Ahmad Fatfat, told the BBC's Newshour programme that the conflict could escalate into civil war. "What Mr Assad is trying to do now is transfer his problem to all the countries around Syria - to Turkey, to Lebanon, to Iraq, to Jordan, and Lebanon is the most fragile in this story," he said. "And maybe Assad will do what he can to transfer Lebanon into a hell situation so he can think later on that what is going on is a general war in the Middle East and not a revolution in Syria." Arab League Secretary General Nabil al-Arabi said he hoped the situation in Lebanon would be settled, with calm returning. "It's very delicate what's going on and I hope the Lebanese, as usual, will be able to get through this difficult time," Mr Arabi said.