Mobs of men belonging to Hezbollah blocked roads with burning tires, garbage cans, and set cars on fire to enforce a strike called to protest government economic policies and to demand higher minimum wages. They fought occasional gun battles with their political opponents, but it was not clear if there were any casualties.

The violence deepened tensions in a country already mired in a 17-month-old political crisis between the opposition led by Hezbollah and supported by Iran and Syria and the government of Prime Minister Fouad Siniora, which is backed by the West and Saudi Arabia. The standoff has left the country without a president since November.

Many of the clashes were in mixed Sunni-Shiite neighborhoods where young men on each side shouted epithets, yelled slogans with clear sectarian undertones and hurled stones at each other. Armed civilians were visible on some streets.

Lebanese military forces raced between neighborhoods in armored personal carriers trying to contain the fighting and shooting in the air to disperse the crowds.

“This is the first day of the civil war,” said a government supporter who gave his name as Omar in the Sunni neighborhood of Tariq al-Jadideh. “They are the aggressors and they will be buried here.”

A few miles away, supporters of Hezbollah were vowing to continue protesting until they bring down the government of Mr. Siniora.

“We are staying here,” said a protestor who gave his name as Abu Rish. “We have money and support from Iran and Syria and we can go on like this for another 50 years.”

Flights to and from Beirut airport were either canceled or delayed as airport workers joined the strike for six hours. As Hezbollah supporters reinforced roadblocks along roads leading to the airport, flights had still not resumed by late Wednesday.

The clashes came a day after the government accused Hezbollah of violating Lebanon’s sovereignty by operating its own private telephone network in south Lebanon and the southern suburbs of Beirut without permission from the authorities and of placing several spy cameras on a road outside the airport.