By Kim Ghattas
BBC News, Beirut
Waving Lebanese flags and chanting anti-government slogans, hundreds of thousands of people took over the heart of the Lebanese capital.
The protesters vow they will not leave until Mr Siniora resigns
It was a massive show of force by the opposition, with Hezbollah helping to mobilise and organise the protesters.
"We want a clean government", said some, while others screamed: "Out, out, Siniora."
In his offices nearby, Prime Minister Fouad Siniora must have been able to hear the crowds.
A heavy army presence blocked all access to the building - the Grand Serail, as the former Ottoman barracks is known.
Speaking outside, in front of armoured personnel carriers, cabinet minister Nayla Moawad told the BBC that if resigning was the solution, ministers would have quit their jobs.
"But this is not a power struggle, for us this is about fighting for the protection of democracy in Lebanon," she said.
But Hezbollah, an ally of Syria and Iran, accuses the Western-backed government of failing the Lebanese and of being a puppet of the Americans.
But it was not just the Shias who were protesting on Friday.
Feb 2005: Former PM Rafik Hariri
June 2005: Anti-Syria journalist Samir Kassir
June 2005: Ex-Communist leader George Hawi
Dec 2005: Anti-Syria MP Gebran Tueni
Nov 2006: Industry Minister Pierre Gemayel
One of Hezbollah's main allies is Michel Aoun, a Christian and former army general. He has a large following, not only among Christians.
Although he fought the Syrians in the 1980s, he has now sided with the pro-Syrian Shia movement against Mr Siniora.
Last year, huge demonstrations by anti-Syrian crowds brought down a government backed by Damascus.
The massive protests were sparked by the killing of the former Prime Minister Rafik Hariri in February 2005.
Mr Hariri was blown up with 22 others in a massive car bomb in central Beirut. His killing was blamed on Syria, which denies any involvement.
But in April, the Damascus government had to withdraw its troops from Lebanon under intense Lebanese and international pressure. Lebanese optimism was short-lived however.
Many of last year's demonstrators say their political leaders have disappointed them. Then, in July and August, the country's infrastructure was devastated by a month of fighting, sparked by Hezbollah's capture of two Israeli soldiers.
Hundreds of people stayed behind to set up tents after the rally
Emboldened by what it saw as a victory over Israel, the Shia group has been pressuring the Siniora government. Last month, with its allies, Hezbollah pulled out of the cabinet, undermining the legality of the government.
Then, last week, the anti-Syrian cabinet minister Pierre Gemayel was killed - the fifth assassination in two years. And, now again, Lebanon is in political ferment.
The huge crowds that showed up at the minister's funeral as well as today, are completely different and show that Lebanon is very polarized.
Both camps are sticking to their guns for now.
In the centre of Beirut, after the end of the rally, hundreds of people stayed behind, setting up tents outside the office of Mr Siniora and vowing they will not leave until he resigns.
In the evening, intense political contacts convinced the demonstrators to free some of the roads leading to the Grand Serail. But the opposition is certainly in it for the long haul.