BEIRUT — Gunmen erected roadblocks, burned tires and fired into the air in downtown Beirut in the predawn hours of Tuesday, while in the suburb of Jounieh to the east, at least two land mines were found on the grounds of a hospital.

The roadblocks were manned by Shiites who apparently support President Bashar Assad, of Syria, and who were angry over the arrest of a Shiite man for firebombing and shooting into the offices of New TV, a Lebanon broadcaster that has been critical of the Syrian government.

The incidents aroused renewed concern that Lebanon's sectarian factions would once again be dragged into the conflicts of its neighbor.

Syria has close ties with Hezbollah and Amal, the dominant Shiite political parties here, and their activists have been strong supporters of Assad.

The firebombing suspect was identified as Wissam Alaaeddine, according to local news reports.

A Facebook page in that name, reported by the Lebanese Broadcasting Corp. TV network, showed photographs of the Syrian president, and news accounts said Alaaeddine was formerly a member of the Amal party.

Security-camera footage at the television station showed masked men pouring gasoline on tires near the entrance and setting them alight. The attackers apparently set one of themselves on fire and one was seen fleeing the scene with his foot aflame.

After Alaaeddine's arrest, gangs of young men, many masked, set up roadblocks in downtown neighborhoods, setting tires on fire and firing into the air.

Karma Khayat, the deputy head of news for New TV, said political leaders from the Hezbollah and Amal parties visited the station after the attack to assure officials there that they had not approved of the attack.

However, she said those who took to the streets after the arrest were supporters of the Shiite parties and their actions were clearly organized.

The land mines were found in a bucket near the entrance of the Our Lady of Lebanon Hospital in Jounieh, in the Christian area east of Beirut.

There were no claims of responsibility.