The Lebanese army hit the streets of Beirut on Monday, a show of force aimed at stopping sectarian attacks and lawlessness in the city following the assassination of top intelligence chief Wissam al-Hassan on Friday.

The Lebanese army regularly sets up checkpoints and sends soldiers out on patrol in the capital during times of instability or heightened security, such as a recent visit by the pope. But Monday’s deployment, as the workweek began, was larger than any in recent months, according to an army official who spoke on the condition of anonymity because he was not authorized to speak on the record.

Shops and cafes in the city opened, but traffic was light amid fears that Lebanon could be on the brink of a new conflict, perhaps fueled by the violence that has engulfed neighboring Syria.

Gun battles broke out in Tariq Jdeideh, a neighborhood where Sunni and Shiite militiamen clash regularly, in the early hours of Monday. Fighting continued throughout the day, and masked Sunni gunmen set up checkpoints on roads around the neighborhood to question those in passing cars, according to the Associated Press.

Soldiers deployed in Tariq Jdeideh to stop the fighting were drawn into clashes with gunmen, according to the army official.

The assassination of Hassan, a prominent Sunni figure, has hit a volatile sectarian fault line at a time when tensions linked to the conflict in neighboring Syria are running high. The Sunni community in Lebanon mostly supports the Syrian opposition, while Shiites mostly support the government of Syrian President Bashar al-Assad.

Many Lebanese blame the Syrian government for the killing of Hassan, who had recently arrested one of Assad’s closest allies in Lebanon on suspicion of organizing a bombing campaign against prominent Sunni figures.

As the army worked to clear the streets of Beirut of gunmen — as well as burned tires and debris left behind by demonstrators protesting Hassan’s killing — it also issued a stern statement. “The developments that have occurred in the last hours proved without a doubt that the country is going through critical moments,” the statement said. “The proportion of tension in some areas is rising to unprecedented levels.”

The statement also warned political leaders not to stoke the violence and noted that “security is a red line.”

In Washington, the State Department said it was concerned about the possibility of violence from Syria spilling across the border to Lebanon and elsewhere. A spokesman, Mark Toner, said an FBI team was headed to Lebanon to assist in the investigation into the bombing that killed Hassan.

Also Monday, the U.N. special coordinator for Lebanon, Derek Plumbly, and ambassadors from the five permanent members of the U.N. Security Council met with Lebanese President Michel Suleiman to express support for the government. Afterward, Plumbly said the group calls on “all parties in Lebanon to preserve national unity.”

Meanwhile, heavy clashes linked to the assassination of Hassan also broke out in the northern city of Tripoli early Monday and continued periodically throughout the day. The fighting was concentrated in the neighborhoods of Bab al-Tebbaneh, which is predominantly Sunni, and Jabal Mohsen, which is predominantly Alawite, the minority Shiite sect to which Assad belongs.

One young girl was killed by a sniper in Tripoli, according to the National News Agency.

Protesters have set up at least eight tents around the residence of Prime Minister Najib Mikati in Tripoli to demand his resignation, according to the Daily Star.

Some Sunnis see Mikati, who is Sunni, as a traitor to their community for serving in a government that was largely cobbled together by the Shiite militia and political party Hezbollah last year.

Mikati offered to resign after the assassination, but Suleiman asked him to remain in his post.

Clashes also broke out near the southern city of Sidon on Monday, leaving at least one person dead, according to the Associated Press.

The clashes Monday came after a violent demonstration Sunday at Hassan’s funeral. A number of protesters, riled up and encouraged by Sunni leaders at the funeral, attempted to storm the Grand Serail in central Beirut, an Ottoman-era palace that serves as the main seat of government in the city.

Riot police fired shots in the air and canisters of tear gas to disperse the crowd.

Suzan Haidamous and Ahmed Ramadan contributed to this report.