Clashes in Beirut and Tripoli have heightened fears Syria's civil war is spreading into Lebanon, pitting local allies and opponents of Syrian president Bashar al Assad against each other.
The Lebanese army promised decisive action to quell the violence, which was touched off by the assassination of a senior intelligence officer last week.
That killing has plunged the country into a political crisis and the army command urged party leaders to be cautious in their public statements so as not to inflame passions further.
It issued the warning after troops and gunmen exchanged fire in Beirut's southern suburbs, while protesters blocked roads with burning tyres.
Many politicians have accused Syria of being behind the killing of Brigadier General Wissam al Hassan, an intelligence chief opposed to the Syrian leadership, who was blown up by a car bomb in central Beirut on Friday.
Opposition leaders want Prime Minister Najib Mikati to resign, saying he is too close to Mr Assad and his Lebanese militant ally Hezbollah, which is part of Mr Mikati's government.
The worst of the clashes took place in the northern port of Tripoli, scene of previous fighting between Sunni Muslims backing the Syrian insurgents and Alawites sympathetic to Mr Assad.
Six people were killed and 50 wounded in fighting between the Sunni district of Tabbaneh and the Alawite Jebel Mohsen. Among the victims was a nine-year-old girl shot by a sniper.
Fighting in Beirut occurred on the edge of Tariq al Jadida, a Sunni Muslim district near Shi'ite Muslim suburbs. Locals reported heavy gunfire between gunmen armed with rifles and rocket-propelled grenades.
Soldiers killed one gunman in Tariq al Jadida, the army said, a Palestinian from a refugee camp who had shot at them.
The violence escalated on Sunday after thousands of people turned out in Beirut's Martyrs' Square for the funeral of Mr Hassan, who was buried with full state honours in an emotionally charged ceremony.
As the funeral ended, hundreds of opposition supporters broke away and tried to storm the nearby government offices, prompting security forces to fire tear gas and shots in the air to repulse them.
The army command said: "We will take decisive measures, especially in areas with rising religious and sectarian tensions, to prevent Lebanon being transformed again into a place for regional settling of scores, and to prevent the assassination of the martyr Wissam al Hassan being used to assassinate a whole country."
Troops in full combat gear and armoured personnel carriers stood guard at traffic inter-sections and government offices, with barbed wire and concrete blocks protecting buildings.
Beirut was noticeably quiet as people stayed at home because they feared being caught in more violence. Many shops, offices and restaurants were shut or empty and the area was free of its normal traffic chaos.
Mr Mikati offered to resign at the weekend to make way for a government of national unity, but Lebanese President Michel Suleiman persuaded him to stay in office to allow time for talks on a way out of the political crisis.
Contextual targeting label: