Nasrallah hits out at governmentExperts' viewsTimelineIn video:James Bays reports on the clashes in Beirut
Hezbollah took control of all roads leading to Beirut's international airport, Lebanon's only air link to the outside world.
According to Elie Zakhour, a port official, Beirut's sea port was also shut down "until further notice" because of the situation, Lebanon's state-run National News Agency reported.
Government forces began taking up positions in some neighbourhoods in west Beirut abandoned by the pro-government groups.
An opposition official said the roadblocks placed by its supporters would not be lifted until the government rescinded its measures against Hezbollah and sat down for a national dialogue.
Another opposition spokesman threatened to attack what he described as the security centres of Walid Jumblatt, leader of the mainly Druze Progressive Socialist Party and a prominent Siniora ally, in the Chouf region of Mount Lebanon.
In a statement to Al Jazeera, Weam Wahab, a former Hezbollah minister, ordered the Lebanese army command to take control of the areas by Friday night, otherwise he would act to remove them shortly.
Tension between the government and Hezbollah sharply 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.
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Lebanese troops evacuated the staff of the TV station's terrestrial and satellite studios in the Kantari area of western Beirut.
Security sources said Hezbollah and allied fighters had overrun offices of Hariri's Future business conglomerate across western Beirut.
The headquarters of the Future media group's Al-Mustaqbal daily was also surrounded by fighters firing rocket-propelled grenades, setting fire to one floor, its managing editor said.
"He said that the government should have undertaken these moves earlier, but predicts that the fighting will end soon," Amin said, referring to Jumblatt."I did not anticipate such a strong response from Hezbollah, but ... yes ... the group is much stronger than other armed militias," Jumblatt said.
He also said: "If you want to know what the next move for Hezbollah will be, ask [Mahmoud] Ahmadinejad [the Iranian president]. This situation goes beyond Lebanese borders."
Normality urged
From the other side of the political spectrum, Michel Aoun, a Christian leader allied with Hezbollah, said that normality should be restored on the streets."The derailed carriage is now back on track. We hope from this point that things will fall back into the normal course [of events]," he said on Friday.Aoun said he had sent a letter to Ban Ki-moon, the UN secretary-general, and various member states of the UN Security Council, but "did not find a clear response to avert the crisis".
On the diplomatic front, Saudi Arabia and Egypt have called for an urgent meeting of Arab foreign ministers to try to halt the violence.
"An emergency meeting of Arab foreign ministers in Cairo to discuss the crisis will be held in two days," Hossam Zaki, the Egyptian foreign ministry spokesman, said on Friday.
The UN Security Council also called for "calm and restraint", urging all sides to return to peaceful dialogue.
Syria, an ally of Lebanon's opposition parties, said the dispute in Lebanon was an "internal affair" and expressed hope the feuding parties would find a solution through dialogue.
Al Jazeera's Mike Hanna, reporting from Jerusalem, said: "The Israeli government does not view the current situation as a threat, but sees it as an internal matter.
"Israel, however, has always held the position that Hezbollah's intent is to split Lebanon."