The demonstration came a day after Syrian and Lebanese leaders announced that Syrian forces would transfer to eastern Lebanon before the nations discuss a full withdrawal.
On Tuesday, two senior Lebanese officials said a major redeployment of the Syrian army from central and northern Lebanon would begin late in the day and be completed by March 23.
Organizers handed out Lebanese flags and directed the men and women to separate sections of the square. Loudspeakers blared militant songs urging resistance to foreign interference. Demonstrators held up pictures of Syrian President Bashar Assad and signs saying, "Syria & Lebanon brothers forever."
Other placards read: "America is the source of terrorism;" "All our disasters are from America;" "No to American-Zionist intervention; Yes to Lebanese-Syrian brotherhood."
Many of the people were bused in from Syria and parts of Lebanon that are under Syrian control, reports CBS News' Edward Yeranian.
"Hezbollah is very well-organized," Middle East analyst Hazhir Teimourian, formerly of the Times of London, told CBS Radio News. "Large numbers of people, whether they like it or not, have been bused into Beirut, so we expected it to be large but it is not necessarily as sincere."
Black-clad Hezbollah guards handled security, lining the perimeter of the square and taking position on rooftops. Trained dogs sniffed for bombs.
Large cranes hoisted two giant red-and-white flags bearing Lebanon's cedar tree. On one, the words, "Thank you Syria," were written in English; on the other, "No to foreign interference."
Hezbollah opposes the U.N. resolution drafted by the United States and France last year calling for Syria to withdraw its 14,000 troops from Lebanon.
Participants in the demonstration in front of the U.N. offices stressed that the foreign influence they referred to was from the United States, France and other countries, not Syria, which they welcomed.
"Syria should not leave. We are one hand and one people," said 16-year-old Esraa Awarki, who traveled by bus from Sharkiya in southern Lebanon. "Why do they want us to split now?"
The demonstration was in front of U.N. offices. Hezbollah opposes the U.N. resolution drafted by the United States and France last year calling for Syria to withdraw its 14,000 troops from Lebanon.
In Washington, President Bush demanded again that Syria pull its troops out of Lebanon and allow free elections. "All Syrian military forces and intelligence personnel must withdraw before the Lebanese elections for these elections to be free and fair," he said Tuesday.
Lebanese Defense Minister Abdul-Rahim Murad told The Associated Press that "all the force in the (central) mountains and north will move to the Bekaa (Valley) as of 10 p.m." He said that included the main Syrian intelligence offices in Beirut.
Deputy Prime Minister Issam Fares, speaking to LBC television, said the full withdrawal to the eastern valley will be complete before the end of March, "or in other words before the (Arab) summit, which falls March 23."
Most of the Syrian troops were still in position Tuesday, with AP reporters in the mountains overlooking Beirut seeing only scattered movement of military trucks heading toward the Bekaa Valley.
A truck carrying 11 soldiers and supplies headed east at midmorning but most of the military traffic was moving in the other direction — empty trucks and buses traveling west apparently to collect soldiers and equipment.
Fares said he believes the next phase, the full withdrawal from Lebanon, will be "speedy."
"The army's movements are unannounced. They are secrets. But in my view it will be fast," he said, without giving a date.
Riad Solh Square in Beirut is just a few blocks from another downtown square where opposition protesters have been rallying for days, demanding that Syria withdraw its troops.
Tuesday's rally was far bigger than the more than 70,000 anti-Syrian protesters who filled nearby Martyrs' Square on Monday. That was the biggest rally yet of anti-Syrian furor, as demonstrators waved Lebanon's cedar-tree flag and thundered, "Syria out!"
There were no independent estimates of Tuesday's crowd, but at least 500,000 people crowded Riad Solh Square and nearby streets. The Lebanese army blocked the road between the two squares with an armored carrier.
"I ask our partners in the country or those looking at us from abroad: Are all these hundreds of thousands of people puppets? Is all this crowd agents for the Syrians and intelligence agencies?" Hezbollah leader Sheik Hassan Nasrallah said to cheers from the crowd.
At least one opposition leader said the pro-Syrian government pressured people to turn out Tuesday and some reports said Syria bused in people from across the border.
Hezbollah, a Shiite Muslim group, has been mobilizing its followers from across the country for the protest, also meant to denounce the U.N. resolution that also called for dismantling militias — a point Hezbollah sees as aimed at its well-armed military wing.
Hezbollah is widely admired both within Lebanon and across the Arab world for driving Israeli forces out of the country's south. It also has the organizational capability and party discipline to mobilize massive street protests, drawing its strength from the Shiite Muslim community, Lebanon's largest religious sect with 1.2 million people.
In the outlying heavily Shiite regions of the Bekaa and the south, loudspeakers had urged followers to travel to Beirut for the protest.
Opposition leaders, who have been courting Hezbollah's support to oust Syrian troops, accused Lebanese intelligence agents of exercising pressure on municipalities, public schools and institutions to drive up the number of demonstrators.
Hezbollah officials denied the charges, saying it is part of a campaign to make the demonstration seem "imposed and involuntary."
Hezbollah, funded by Iran and backed in part by Syria, has emerged as a key player in the latest political instability, capable of tilting the balance either in favor of the government or the opposition.
Cabinet Minister Talal Erslan drew cheers Tuesday when he said the crowd came from all over Lebanon "to affirm our gratitude to Syrian president Bashar Assad."
"We have come here to affirm Lebanon's independence, sovereignty and unity ... and say no to the flagrant foreign interference in our affairs," he said.
At one point, the crowd observed a moment of silence for former Prime Minister Rafik Hariri, whose assassination in a Feb. 14 bombing triggered the weeks of anti-Syrian demonstrations. Many Lebanese accuse Syria and Lebanon's government of responsibility for Hariri's death; both deny any involvement.
Faced with incessant international pressure and raging Lebanese opposition, Assad on Saturday announced his troops would withdraw after nearly three decades in Lebanon. On Monday, he met with President Emile Lahoud in Damascus and jointly announced a plan.
But the plan set no deadline for the complete withdrawal of Syrian troops and intelligence agents from Lebanon, and Washington rejected the pullback as insufficient. The plan also was unlikely to satisfy the Lebanese opposition and the rest of the international community.
Syria has had troops here since 1976, when they were sent as peacekeepers during Lebanon's 1975-90 civil war. When the war ended, the troops remained and Syria has dominated Lebanon's politics ever since.