The UN has called for the protests and fighting to cease immediately
At least seven people have been killed in fighting between supporters of Lebanon's government and followers of the Hezbollah-led opposition in Beirut.
Clashes continued through the night as gunmen fought with rifles and grenade launchers in central and southern areas of the city.
Fighting erupted after Hezbollah leader Hassan Nasrallah condemned the closure of its telecommunications network.
He said the move by the government was a "declaration of war".
Mr Nasrallah vowed to "cut off the hand" that attempted to dismantle it and said that the only way out of the crisis was for the government to rescind the decisions it took on Monday.
Earlier, the Lebanese army command warned its unity was at risk if the ongoing political crisis and civil unrest in Beirut continued.
In recent years, it has been seen as one of Lebanon's most neutral institutions, but correspondents say clashes between rival factions could draw it into the conflict.
For most of Thursday there was a tense stand-off in the sensitive areas of central and south Beirut, where Sunni and Shia communities overlap and where gun battles had broken out the previous day.
Gunfire on the streets of Beirut
But shortly after the televised speech by Sheikh Nasrallah ended, the sound of heavy gunfire and explosions could be heard echoing through the streets of the capital.
The fighting was still going on long after nightfall in the districts of Corniche al-Mazraa and Ras al-Nabaa, where a mother and son were killed when a rocket-propelled grenade hit their apartment and another man was shot dead, officials told the Associated Press.
A member of the parliament's security force was also killed near the residence of the Shia speaker, Amal leader Nabih Berri, during clashes nearby, officials said.
A woman was also killed by a stray bullet in the southern suburb of Haret Hreik, a Hezbollah stronghold, following Sheikh Nasrallah's speech, they added.
Hezbollah fighters and their allies reportedly tried during the clashes to take over buildings in which there are offices of the Sunni Future movement led by Saad Hariri, the leader of the pro-Western parliamentary majority.
Hezbollah needs to make a choice: be a terrorist organisation or be a political party, but quit trying to be both
Later, Mr Hariri demanded the fighting stop immediately and gunmen from both sides pulled off the streets in order "to save Lebanon from hell".
"My appeal to you is to stop the language of arms," he said.
Mr Hariri also said Hezbollah should lift its "siege" of Beirut, and called for a meeting with Sheikh Nasrallah as soon as possible.
Mr Hariri then proposed a compromise on the government decision to close down Hezbollah's telecommunications network, calling it a "misunderstanding" and saying the army would have the final call.
On Tuesday, the government declared the group's fixed-line network covering its strongholds of south and east Lebanon, and southern Beirut, illegal, saying it was a threat to state security.
But Sheikh Nasrallah earlier explained how his group's military wing regarded the network as "the most important part" of its defensive measures and warned that any attempt to dismantle it would be resisted.
The main roads to the international airport are blocked by barricades
"This decision is first of all a declaration of war and the launching of war by the government... against the resistance and its weapons for the benefit of America and Israel," he said.
Beirut has been largely paralysed by roadblocks set up over the past two days by opposition supporters, led by Hezbollah, during protests against the government's decision, which started as a strike over pay.
UN Secretary General Ban Ki-moon has urged both sides "to cease immediately these riots and to re-open all roads in the country".
The US government also called on Hezbollah and the opposition to "start playing a constructive role and stop their disruptive activities".
"Hezbollah needs to make a choice - be a terrorist organisation or be a political party, but quit trying to be both," a spokesman said.
Lebanon is currently witnessing its deepest political crisis since the civil war and has been without a president for five months.
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