BEIRUT, Lebanon — Sectarian fighting spread through the streets of Beirut on Thursday as Shiite Hezbollah supporters and the Lebanese government’s Sunni backers battled with machine guns and rocket-propelled grenades shortly after Hezbollah’s leader vowed to fight any attempt to disarm his men.

The violence first erupted in Muslim West Beirut, where masked gunmen on street corners opened fire along Corniche Mazraa, a major thoroughfare that has become a demarcation line between the two sides. There was also fighting in the nearby Ras el-Nabeh area. There was no immediate word on casualties.

The violence spread to Khandaq el-Ghamiq, a neighborhood adjacent to downtown, which is home to the government’s offices. Shootings and explosions were reported by witnesses and television stations in the Aisha Bakkar neighborhood near the office of Lebanon’s Sunni spiritual leader, who is allied with the government. Gunfire and explosions were also heard in a nearby district where the opposition-aligned parliament speaker has his official residence.

Troops in armored carriers had earlier moved in to West Beirut to separate people who were trading insults and throwing stones at each other, but the troops did not attempt to stop the street battles that then broke out.

The army, which has been struggling to contain the disturbances, warned of the consequences to the country and the military.

“The continuation of the situation as is is a clear loss for all and harms the unity of the military institution,” a statement said.

The army has largely stayed out of the broader political struggle between Hezbollah and the government for fear of exacerbating the situation. The army’s commander is the two factions’ consensus candidate for president.

Gen. Michel Suleiman so far has advised the government not to declare a state of emergency.

The tensions in Lebanon are partly fueled by the growing rivalry beween Shiite Iran and Sunni Arab countries like Saudi Arabia and Egypt, which back the government.

Many worry that the conflict could push Lebanon back toward the lawless days of its 1975-1990 civil war, when militias ran entire neighboroods and battled over territory, religious or political affiliation. The conflict left 150,000 dead and wrecked entire areas of the capital.

The latest clashes came close on the heels of a defiant speech by Hezbollah leader Hassan Nasrallah, who said his Iranian-backed militant organization would respond with force to any attacks.

“Those who try to arrest us, we will arrest them,” he said. “Those who shoot at us, we will shoot at them. The hand raised against us, we will cut it off.”

It was the second day of fighting that has turned some city neighborhoods into battlegrounds and spilled over to other parts of the country.

The violence appeared to begin as a test of wills between political rivals who have been locked in a 17-month power struggle for control of the government. It now could be degenerating into a wider and deadlier sectarian conflict, with the Sunnis’ spiritual leader denouncing Hezbollah and appealing to a largely Sunni Islamic world to intervene.

The rivals have failed to agree on electing a president, leaving the country without a head of state since November.