The Lebanese army has deployed on the streets of Beirut and Tripoli in a bid to calm deadly tensions.
Several people were killed in gunfights between pro- and anti-Syrian factions in Tripoli on Monday after the death of a senior security official on Friday.
European Union foreign policy chief Catherine Ashton has been in Beirut for talks with President Michel Suleiman about the murder of Wissam al-Hassan.
She also met Prime Minister Najib Mikati in an apparent show of support.
Former Prime Minister Fouad Siniora, a leader of the Western-backed 14 March opposition bloc, was also expected to meet the president later.
Over the weekend, Mr Suleiman rejected an offer of resignation from Mr Mikati - a Sunni Muslim who heads a cabinet dominated by the pro-Syrian Shia Islamist movement Hezbollah and its allies.
The decision prompted Mr Siniora to warn: "The Lebanese people won't accept, after today, the continuation of the government of assassination."
He made the remarks at the funeral of the security official killed on Friday, Gen Wissam al-Hassan, who had close links to the 14 March alliance and the family of its leader, former Prime Minister Saad Hariri. Opposition MPs were on Tuesday boycotting parliamentary sessions.
On Monday, the army urged "all political leaders to be cautious when expressing their stances and opinions" and in attempting to mobilise public action "because the fate of the nation is at stake".
The army is a widely respected institution in Lebanon that has often been required to stand between the country's diverse political and religious factions.
The violence comes after the funeral of Gen Hassan - a Sunni and the head of the intelligence branch of the Internal Security Forces (ISF).
He was an outspoken critic of Syrian President Bashar al-Assad - who is a member of the Alawite sect, an offshoot of Shia Islam - and was killed in a car bomb on Friday, along with his bodyguard and at least one other person.
Opposition figures blamed the attack on the Syrian government.
On Monday, dozens of people set up camp outside Mr Mikati's office, calling for his resignation.
Gen Hassan led an investigation into the assassination in 2005 of Mr Hariri's father, Rafik, which implicated Damascus.
He also organised the recent arrest of Michel Samaha, a former minister accused of planning a Syrian-sponsored bombing campaign in Lebanon.
Syrian troops withdrew from Lebanon in the wake of Rafik Hariri's murder, ending a 29-year military presence.
Thousands attended Gen Hassan's funeral on Sunday, which became a political rally against both Mr Mikati and Syria.
Police scuffled with a group of protesters who attempted to storm the prime minister's office, and overnight into Monday protesters set up road blocks in Beirut, prompting exchanges of gunfire.