BEIRUT (AP) -- Lebanese troops launched a major security operation on Monday 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 overnight killed at least two people.

Opponents of Syria have blamed the regime in Damascus for the killing of Lebanese Brig. Gen. Wissam al-Hassan in a Beirut car bomb on Friday. With Lebanon already tense and deeply divided over the civil war next door, the assassination has threatened to drag the country back into the kind of sectarian strife that plagued it for decades _ much of it linked to Syria.

Sporadic cracks of gunfire rang out in Beirut as soldiers backed by armored personal carriers with heavy machine guns took up position on major thoroughfares and dismantled roadblocks.  At times, troops exchanged gunfire with Sunni gunmen.

Al-Hassan was a Sunni who challenged Syria and its powerful Lebanese ally, the Shiite militant group Hezbollah. The uprising in Syria is dominated by the Sunni majority fighting Syrian President Bashar Assad, who like many who dominate his regime, is a member of the Alawite sect _ an offshoot of Shiite Islam. Lebanon and Syria share similar sectarian divides that have fed tensions in both countries, increasingly.

Most of Lebanon's Sunnis have backed Syria's mainly Sunni rebels, while Lebanese Shiites tend to back Assad.

Al-Hassan's assassination has imperiled Lebanon's fragile political balance. Many politicians blamed Syria for the killing and angry protesters tried to storm the government palace after al-Hassan's funeral on Sunday, venting their rage at leaders they consider puppets of a murderous Syrian regime. But were pushed back by troops who opened fire in the air and fired tear gas.

Prime Minister Najib Mikati, a Sunni, told As-Safir newspaper that when he took up his post last year, he intended to protect all Lebanese, particularly Sunnis.

"I was convinced that through this mission, I am protecting my country, my people and especially fellow members of my sect."

The prime minister of Lebanon is usually a Sunni according to a sectarian division of top posts in the state. Over the past year, pro-Syrian Hezbollah and its allies have come to dominate the government.

On Sunday night, a group of anti-Syrian protesters started an open-ended sit-in outside Mikati's house in his hometown of Tripoli. The protesters said they will only end the sit-in when Mikati resigns.

Ambassadors of Britain, the U.S., Russia, China and France and the U.N. Special Coordinator for Lebanon met President Michel Suleiman to express support for him.

"The permanent members at the United Nations call upon all the parties in Lebanon to preserve stability," Derek Plumbly, the U.N. representative, told reporters in Arabic while surrounded by the five ambassadors. "We strongly condemn any attempt to shake Lebanon's stability."

Overnight, Sunni and Shiite gunmen clashed in two Beirut neighborhoods and officials also reported heavy clashes late Sunday and early Monday in the northern city of Tripoli and towns between the capital Beirut and the southern city of Sidon.

The officials, speaking on condition of anonymity because they were not authorized to talk to the media, said a man was killed in shooting in the Wadi Zayneh area north of Sidon and another person died in the Tripoli clashes. The officials said the clashes wounded at least six people in Beirut and 10 in Tripoli.

An Associated Press photographer saw dozens of gunmen roaming the streets on Monday in Beirut's predominantly Sunni neighborhood of Tariq Jadideh, where much of the fighting has taken place. Local Sunni leaders were calling the gunmen by telephone urging them to pull out of the streets.

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.

A woman who lives in the neighborhood said the fighting began shortly after midnight and lasted until sunrise.

"We couldn't sleep because of the shooting. There were also some booms," she said, referring to rocket-propelled grenades. She asked that her name not be used for fear of reprisals.

In Tripoli, residents said scores of soldiers deployed around the city in an attempt to bring back calm. The military also set up checkpoints, searched cars and asked people for identity cards.