At least five people have been killed in fierce fighting between supporters of Lebanon's government and the opposition in Beirut, officials say.
Television showed gunmen firing rifles and rocket-propelled grenades in central and southern areas of the city.
The fighting began after the leader of Hezbollah described the government's move Fierce clashes resume in Beirut ...
Gunfire and Explosions in Beirut as Lebanon crisis continues ...
Hezbollah chief threatens Israel ... to close its telecommunications network as a "declaration of war".
Hassan Nasrallah vowed to "cut off the hand" that attempted to dismantle it.
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.
'Siege'
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.
But shortly after the televised speech by Sheikh Nasrallah ended, the sound of heavy gunfire and explosions could be heard echoing through the centre of the city. It was still going on after nightfall.
Reports say Hezbollah fighters and their allies are trying 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.
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.
"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.
(BBC)
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