Hundreds of thousands of Lebanese are taking part in the latest protest to press the government to cede more power to the opposition or step down.
Led by Hezbollah and its pro-Syrian allies, the rally is possibly the largest demonstration Beirut has seen.
Christian opposition leader Michel Aoun, a Hezbollah ally, urged the prime minister to accept a national unity government or face further action.
PM Fouad Siniora has vowed to resist what he has called an attempted coup.
Demonstrators have been waving the red, green and white flag of Lebanon, but others are also waving the yellow flag of Hezbollah emblazoned with a fist holding up an AK-47.
Fully-veiled Shia women, Christian students wearing t-shirts and fathers hoisting children on their shoulders were among crowds who cheered a series of opposition speakers urging the government's resignation, the AFP news agency reported.
Mr Aoun, in a video link speech that appeared on giant screens, told the crowd that the time of Mr Siniora's Western-backed government was over.
He said the opposition was committed to "peaceful means, but even other means are legitimate".
Lebanese army combat troops sealed off major roads around the two squares where the rally is taking place.
However there is also a heavy presence of Hezbollah security agents wearing white caps and carrying walkie-talkies, says the BBC's Kim Ghattas in Beirut.
The Hezbollah-led protests began on 1 December.
"We will stay for days, weeks or months. Whatever it takes to bring down the government," one protester, Nader Hafez, told Reuters news agency.
The opposition accuses the government of being weak and corrupt, and says it no longer represents the Lebanese people after six pro-Syrian ministers resigned last month.
Under the constitution, the death or resignation of another two ministers would automatically bring it down.
Hezbollah has been demanding a bigger share in the cabinet that would give it the power to veto government decisions.
Opposition supporters have camped out in Beirut since 1 December
The current cabinet came to office last year in the first election held after the withdrawal of Syrian troops originally stationed in Lebanon during the civil war.
Syria was forced to withdraw its military presence after massive street protests and international pressure, triggered by the February 2005 assassination of former Lebanese Prime Minister Rafik Hariri.
A UN investigation has implicated several Syrian officials in the killing, although Syria has denied any involvement.
The government in Beirut has also accused Damascus of ordering the assassination on 21 November of anti-Syrian cabinet minister Pierre Gemayel.