A rocket-propelled grenade also hit the fence of the heavily protected residence of Saad al-Hariri, the Sunni politician and leader of the governing coalition, in the suburb of Koreitem, a Muslim area of western Beirut on Friday.
Al-Hariri was believed to be inside at the time but unhurt.
Gunmen loyal to Hezbollah also forced Future News, an al-Hariri-owned TV station, off the air in Beirut.
"Armed gunmen surrounded the building, stormed into the garage and demanded that the army shutdown the station," a senior official at the station, said.
The security sources said Hezbollah and fighters from the allied Amal movement - both Shia groups - had overrun offices of al-Hariri's Future group across the predominantly Muslim western half of the Lebanese capital.Al Jazeera's Rula Amin, reporting from Beirut, said: "This is a significant move considering that the Saudi government is a staunch supporter of the ruling coalition in Beirut."
The headquarters of the Future movement's Al-Mustaqbal newspaper was also surrounded by gunmen firing rocket-propelled grenades, setting fire to one floor, its managing editor said.
Nadim Munla, the general manager of Future television, told Al Jazeera that masked armed men entered the control rooms of the television network and cut off the cables.
He said: "We have been effectively prevented from broadcasting and doing our jobs as media professionals.
"I would have to congratulate Hezbollah...They have proven that the gun is stronger than the value of the opinion. We have only one thing left - free speech, and their guns will not silence us."
Reports have also emerged that the Saudi ambassador to Lebanon advised Fouad Siniora, the prime minister, to step down.
"The Saudis see this as a dangerous situation that can escalate rapidly."
Amin also conducted an exclusive interview with Walid Jumblatt, head of the Socialist Progressive Party, member of parliament, and leader of the country's Druze community.
She said that Jumblatt did not regret his backing to remove the head of the country's airport security, whom the government accuses of being too close to Hezbollah.
"Jumblatt did not anticipate such a strong response from Hezbollah, and he is resigned to the fact that the group is much stronger than other armed militia," she said.
"He also said that the government should have undertaken these moves earlier, but predicts that the fighting will end soon."
In several neighbourhoods across the capital, automatic rifle fire could be heard in the worst domestic fighting since the 1975-90 civil war.
Hezbollah also took control of all roads leading to Beirut's international airport, Lebanon's only air link to the outside world.
| Opposition fighters took rapid control|
of many suburbs [AFP]
According to Elie Zakhour, a port official, Beirut's port was also shut down.
Tension between the government and Hezbollah escalated when the cabinet said the group's private phone network was illegal and an attack on the country's sovereignty.
Hezbollah said it was infuriated by government allegations it was spying on Beirut airport and by the cabinet's decision to fire the head of airport security.
Call for restraint
The fighting has prompted urgent appeals for calm from the international community.
Saudi Arabia called for an urgent meeting of Arab foreign ministers to try to halt the violence.
"In light of the dangerous escalation of the situation on the Lebanese scene, the kingdom of Saudi Arabia supports holding an urgent and extraordinary meeting of the Arab League ministerial council in Cairo to discuss the Lebanese crisis and its fallout," a foreign ministry official was quoted by the state SPA news agency as saying on Friday.
The UN Security Council also called for "calm and restraint", urging all sides to return to peaceful dialogue.