Hezbollah, the most powerful Shiite Muslim group that fought a war against Israel over the summer, joined with its pro-Syrian allies to organize the protest against Saniora's Western-backed government. Hezbollah, which is backed by Syria and Iran, had threatened to protest if the government continued to refuse to give it and its allies a veto share of the cabinet.
Waving red and white Lebanese flags and banners with the opposition's motto "We want clean government," the crowd chanted, "Saniora out, we want a free, free government."
The crowd -- estimated by police at 800,000 and at 1 million by Hezbollah -- trapped Saniora and most of his ministers in their offices. Heavily armored security forces surrounded the complex using barbed wire and metal barriers as Saniora continued with his schedule despite the crowd.
On Thursday, Saniora vowed that his government would not fall to the opposition saying that "Lebanon's independence is threatened and its democratic system is in danger." His supporters accuse Syria of trying to regain influence in Lebanon after Syrian troops were ousted from Lebanon following the assassination of former Prime Minister Rafik Hariri in February 2005.
Hezbollah's deputy leader Sheik Naim Kassim said the opposition was fighting against "American tutelage." In a television address to Saniora on Friday, Kassim said, "We will not let you sell Lebanon, we will protect the constitution and people of Lebanon."
Hezbollah's resistance to Israeli attacks in the 34-day war strengthened the group's support among Shiites and encouraged it to demand more political power. For Friday's protest, however, Hezbollah wanted to depict the demonstration as the voice of all Lebanese against the government.
The Lebanese government's failure to back Hezbollah against Israel led to a conflict within the government that includes Hezbollah and other opposition members. In Lebanon, Sunni Muslims generally support Saniora, a Sunni, while Shiites support the pro-Syrian opposition. Lebanon's Christians are divided.
To add to the tensions, six opposition members resigned from the cabinet last month and on Nov. 21, anti-Syrian Christian cabinet member Pierre Gemayel was assassinated. Gemayel's assassination -- blamed by his supporters on the government of Syria -- prompted a large demonstration in Beirut's Martyr's Square.
A senior opposition member told Reuters that the group plans to impose a blockade on the government offices with a sit-in. "Tents are being put up for protesters to encircle the government's headquarters to stop movements to and from it until Saniora falls," he said.
"We're here to bring down the government," said opposition supporter Najwa Bouhamdan. "We, the resistance, don't want any influence from the United States."