Running gunbattles have raged in parts of Beirut after the leader of Hezbollah accused Lebanon's Western-backed government of declaring war on his Shiite militant group.
At least four people were killed and eight wounded in the capital.
In a grim reminder of Lebanon's devastating 1975-90 civil war, factions threw up roadblocks and checkpoints dividing Beirut into sectarian enclaves on the second day of clashes yesterday between Sunni Muslims loyal to the government and Shiite supporters of Hezbollah.
A top Sunni leader went on television urging Hezbollah to pull its fighters back and "save Lebanon from hell".
The army, which has stayed out of the sectarian political squabbling that has paralysed the country for more than a year, did not intervene in the battles.
The chattering of automatic weapons and thumps of exploding rocket-propelled grenades echoed across Beirut into the night.
People huddled in hallways and stairwells as gunmen rushed from one street corner to the next firing at their foes. Some families fled to neighbourhoods that remained quiet.
"There is so much shooting and explosions outside. Our building is in the middle of the fighting," a terrified woman, Ghada Helmi, said.
Fighting began along Corniche Mazraa, an avenue separating Shiite and Sunni areas, then spread to other districts. Combat was heard near the office of Lebanon's Sunni spiritual leader, an ally of the government, and near the official residence of the opposition-aligned parliament.
Fighting intensified minutes after Hezbollah leader Hassan Nasrallah made a televised address charging that the government had declared war on his group when it decided this week to shut down Hezbollah's private telecommunications network.
He warned against trying to disarm Hezbollah and said his fighters would retaliate swiftly if attacked.
"Those who try to arrest us, we will arrest them. Those who shoot at us, we will shoot at them. The hand raised against us, we will cut it off," Mr Nasrallah said in a news conference via video link from his hiding place.
Later in the day, Sunni politician Saad Hariri made a televised appeal to Mr Nasrallah seeking to calm the conflict.
"My appeal to you and to myself as well, the appeal of all Lebanon, is to stop the slide toward civil war, to stop the language of arms and lawlessness," said Mr Hariri, son of former Prime Minister Rafik Hariri, who was assassinated in 2005.