Gunmen from the Shia group Hezbollah have seized most of western Beirut in a third day of fighting between opposition and government supporters.
The Western-backed governing coalition has described it as a "bloody coup". At least 11 people have been killed.
The gunmen, who also back Hezbollah's Shia opposition allies, have forced the closure of pro-government media.
The US has restated its support for Lebanon's government and says Hezbollah is killing innocent civilians.
US Secretary of State Condoleezza Rice said Washington was committed to helping the government of Prime Minister Fouad Siniora.
"We will stand by the Lebanese government and peaceful citizens of Lebanon through this crisis and provide the support they need to weather this storm," she said in a statement.
The opposition has said Hezbollah and its allies will maintain roadblocks around Beirut until there is a solution to the political crisis.
The fighting was sparked by a government move on Monday to shut down Hezbollah's telecoms network.
Lebanon's governing coalition said it was a coup aimed at restoring the influence of Syria and Iran.
"The armed and bloody coup which is being implemented aims to return Syria to Lebanon and extend Iran's reach to the Mediterranean," it said in a statement.
Civil war fears
The UN Security Council has urged the rival parties to stop fighting amid fears of civil war breaking out.
Future TV News: Attacked by gunmen
Hariri residence: Fence hit by rocket-propelled grenade
Siniora office: Prime minister reportedly holed up with staff
Al-Mustaqbal: Newspaper office partially set on fire
Hamra Street: March by armed Shia militants
Lebanon was plunged into civil war from 1975-90, drawing in Syria and Israel, the two regional powers.
Analysts say the key to avoiding such a conflict this time may be the neutrality of the army, and its ability to withstand the sectarian tensions.
The Lebanese army command has warned its unity is at risk if the crisis in the capital drags on.
Syrian President Bashar al-Assad, whose country long dominated neighbouring Lebanon, said on Friday that the political crisis there was an "internal matter".
Having withdrawn its army from the country in 2005, Syria denies meddling in Lebanon's internal politics.
But Damascus has been accused of involvement in the assassination over the past three years of several anti-Syrians, including Rafik Hariri, a former prime minister.
Lebanon has been without a president since late 2007, amid deadlock between the ruling coalition and Hezbollah-led opposition over the make-up of the government.
As residents of west Beirut fled on Friday, the Italian government was reported to be drawing up an evacuation plan for any of its nationals wanting to leave the city.
Earlier, media offices owned by Saad Hariri, a leader of the governing coalition, were shut after being attacked by militants loyal to Hezbollah.
The army moved in after gunmen besieged TV station Future News and partially set fire to the offices of al-Mustaqbal newspaper. Mr Hariri's radio station was also silenced.
'Save Lebanon from hell'
A compromise was reached for the premises to be taken over and protected by the Lebanese army at the price of going off the air.
Lebanese army on the streets of Beirut
Several Sunni neighbourhoods in west Beirut, considered strongholds of Lebanon's ruling bloc, have reportedly been over-run by militants from Hezbollah and its Shia ally Amal.
A rocket-propelled grenade hit the fence of the heavily protected home of Mr Hariri in western Beirut's Koreitem neighbourhood, officials said.
Mr Hariri - Lebanon's top Sunni politician - was thought to have been inside at the time.
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Mr Siniora was reportedly holed up in his heavily guarded offices along with several ministers in downtown Beirut.
The urban warfare has shut down Lebanon's seaport and all but closed the international airport, with burning barricades on major roads in Beirut.
The BBC's Jim Muir in the city says it all amounts to a humiliating blow to the government.
It appears to have badly overplayed its hand in moving to close Hezbollah's telecoms network on Tuesday, he says.
Hezbollah leader Hassan Nasrallah has called the move a "declaration of war".
Mr Hariri said it was a "misunderstanding" and urged gunmen from both sides to withdraw "to save Lebanon from hell".