The violence deepened tensions in a country already mired in a 17-month-old political crisis pitting the Iranian- and Syrian-backed Hezbollah against the government of Prime Minister Fuad Saniora. The troubles have left the country without a president since November.

Explosions and gunfire echoed throughout the city. The cause of the explosions was not immediately known. Few injuries were reported and most were from stone-throwing.

An Associated Press photographer saw gunmen from Hezbollah and its allied Shiite Amal group shooting toward a building that houses a pro-government Sunni group. Police also were seen firing toward a building and at times into the air to try to disperse the crowds.

Labor unions had called for the strike after rejecting a last-minute pay raise offer by the government. Instead, it turned into a showdown between Hezbollah and the government.

The clashes began when government and opposition supporters in a Muslim sector of Beirut exchanged insults and began throwing stones at each other. Witnesses said security forces intervened.

A cameraman for Hezbollah's al-Manar television was beaten by a soldier, the station reported. The state-run National News Agency reported that he was struck in the forehead during the clash.

Bystanders wrapped a shirt on his head to stop the bleeding before he left on his motorcycle.

A soldier was hit in the mouth by a stone and two other news photographers also were hurt by stones, according to witnesses and television reports.

Earlier in the same area, a stun grenade thrown into a crowd lightly injured three protesters and two soldiers, the state news agency reported. It was not immediately clear who threw the grenade.

The clashes spread to several mixed Sunni-Shiite neighborhoods, with Sunnis backing the government and Shiites supporting the opposition. Armed civilians appeared on some streets.

Troop reinforcements raced in armored carriers from one neighborhood to another to contain the disturbances.

Around the city, protesters blocked roads with burning tires, dirt, old cars and garbage cans to protest against government economic policies and demand pay raises.

The protests and labor strike paralyzed Beirut's airport. Employees stopped working for six hours and flights were delayed or canceled while opposition protesters blocked roads to the country's only air facility. Flights later resumed.

Lebanon's political crisis took a turn for the worse this week when the government's Cabinet on Tuesday said it would remove Beirut airport's security chief over alleged ties to Hezbollah.

The government also declared that a telecommunications network used by Hezbollah for military purposes was illegal and a danger to state security.

Hezbollah and Shiite leaders rejected the government's decisions, raising tensions ahead of the planned labor strike.

Hezbollah is listed as a terrorist group by the United States. It has fought Israel for more than two decades, most recently in the 2006 war, and enjoys wide support among Lebanon's 1.2 million Shiites, believed to be the country's largest sect.

The political crisis has exacerbated the country's economic problems. Rising oil prices and a weakening U.S. dollar, the favored currency here, have driven up the cost of living.

Just as the country is divided politically into opposition and pro-government camps, the unions were split as well on whether to support the strike. In Shiite sectors of the city where Hezbollah support is high, the strike was widely observed, with most businesses closed and streets empty.

In areas where government support is strong, some businesses were open but many people stayed off the streets and traffic was lighter than usual amid a heavy army presence.

The U.S. Embassy advised Americans to avoid areas where protests were going, to take "reasonable" security precautions and maintain a low profile in public.