Lebanese troops launched a major security operation to open all roads and force gunmen off the streets, trying to contain an outburst of violence set off by the assassination of a top intelligence official who was a powerful opponent of Syria. Sectarian clashes killed at least six people.
Gunfights have broken out in several areas of Lebanon following Sunday's funeral of anti-Syrian intelligence official Wissam al-Hassan, raising fears that his assassination will further destabilise the country.
On Monday, clashes between Lebanese troops and unidentified armed men took place in Beirut, the capital.
Several people were wounded after the army made a pre-dawn sweep through the district of Tariq Jdideh in pursuit of armed men, and automatic weapons and anti-tank rocket fire could be heard.
The worst of the clashes since late Sunday took place in the northern city of Tripoli, the scene of previous fighting between Sunni Muslims backing the Syrian opopsition fighter and Alawites sympathetic to Syrian President Bashar al-Assad.
The two sides exchanged rocket and gunfire, residents said. Among the victims were a nine-year-old girl shot by a sniper.
Opposition leaders have urged their supporters to refrain from any more violence.
"We want peace, the government should fall, but we want that in a peaceful way. I call on all those who are in the streets to pull back," former prime minister and opposition leader Saad al-Hariri said on the Future Television channel on Sunday evening from Saudi Arabia, where he has been living for over a year.
Al Jazeera's Rula Amin reported from Beirut that security officials were very concerned about the heightened tensions.
"The army's saying that it will act very firmly and concisely to try to confront this phase," she said. "It says Lebanon is going through a very critical time and that tensions have reached unprecedented levels."
Our correspondent went on to say that protesters have set up camp outside the parliament building in downtown Beirut, but it had generated very little momentum. "It does not have the backing of a lot of the March 14 politicians," she said.
The army warned that targeting public or private institutions crossed a "red line", and it would respond.
"The last few hours have proven without a doubt that the country is going through a decisive and critical time and the level of tension in some regions is rising to unprecedented levels," a statement issued by the army leadership said on Monday.
"We call on all political leaders to be cautious when expressing their stances and opinions... because the fate of the country is on the line", the strongly worded statement said.
While pledging to leave politicians to find a political solution to the crisis triggered by Friday's assassination of Hassan, the army also pledged "to stamp out any security violation and safeguard national peace".
"The army leadership reiterates, by action not words, that security is a red line, and likewise the targeting of official institutions and violation of private and public property," it said.
Protesters overnight blocked roads in Beirut with burning tyres, including the highway to the airport.
The capital was noticeably quieter than normal on Monday. Many people stayed at home for fear of violence and the streets were free of the usual traffic chaos.
Hassan was the head of the intelligence branch of the Internal Security Forces and an outspoken critic of Syrian President Bashar al-Assad.
He was killed by a car bomb along with his bodyguard and a civilian in a busy street in the mainly Christian district of Ashrafiyeh.
Thousands of people attended Hassan's state funeral in Beirut, which rapidly became a political rally against both Lebanese Prime Minister Najib Mikati and Syria.
Many politicians have accused Syria of being behind the killing and angry protesters tried to storm the government palace after Hassan's funeral on Sunday.
Opposition leaders and their supporters want Mikati to resign, saying he is too close to Assad and his Lebanese ally Hezbollah, which is part of Mikati's government.
Mikati did offer to stand down on Saturday, but President Michel Suleiman asked him to stay on in the national interest.
Memories are still vivid in Lebanon of the death and destruction of Lebanon's 1975-90 civil war. Heavily-armed soldiers and police were out in force at street junctions and government buildings.