Lebanese army troops launched a major security operation to open roads across the country and force gunmen off city streets today.
Clashes in Beirut and other parts of Lebanon killed at least two people and left more than a dozen wounded overnight.
The latest outbreaks of violence reflected rising tensions between Lebanese groups that support or oppose the regime of Syrian President Bashar Assad in the wake of the assassination last week of top anti-Syrian intelligence official General Wissam al-Hassan.
Most of Lebanon's Sunnis have backed Syria's mainly Sunni rebels, while Lebanese Shi'ites tend to back President Assad.
The Syrian president, like many who dominate his regime, is a member of the Alawite sect, an offshoot of Shi'ite Islam.
The assassination of Gen Hassan has threatened to shatter Lebanon's fragile political balance.
Many politicians have blamed Syria for the killing and angry protesters tried to storm the government palace after Gen Hassan's funeral on Sunday but were pushed back by troops who opened fire in the air and fired tear gas.
Overnight, Sunni and Shi'ite gunmen clashed in two Beirut neighbourhoods.
Officials also reported heavy clashes late on Sunday and early today in Tripoli and several towns between Beirut and Sidon.
A man was killed in shooting in the Wadi Zayneh district of Sidon and another died in the Tripoli clashes.
At least six people were wounded in Beirut and 10 in Tripoli.
In some roads around Tariq Jadideh, masked Sunni gunmen set up checkpoints, stopping cars and asking people about their destination and where they were coming from.
Fighting began shortly after midnight and lasted until sunrise.
In Tripoli, residents said scores of soldiers had been deployed around the city. The military also set up checkpoints, searched cars and asked people for identity cards.
In the midst of the unrest the youth wing of the March 14 coalition called for an evening rally in Beirut's Martyrs' Square.
It claimed that the rally would be peaceful.
The March 14 movement is a coalition of groups ranged against the influence of the Syrian government that emerged after the 2005 assassination of former prime minister Rafik Hariri.
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