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."
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.
"The Hezbollah and their supporters know that if the Syrians are forced by the United Nations, by the rest of the world, to end their occupation of Lebanon, the Hezbollah would lose their freedom to act as they have done for so many years," said Teimourian.
Hezbollah's leader, Sheikh Hassan Nasrallah, told the massive crowd of half a million cheering demonstrators that the majority of Lebanese people support their country's historic ties with Syria and refuse any U.S. intervention in their country, reports Yeranian. He hurled insults at both the U.S. and President Bush, saying Israel should not expect to benefit from chaos in Lebanon.
The square was 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 the 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!"
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.
A day after the Syrian and Lebanese leaders announced that Syrian forces would redeploy to eastern Lebanon before the nations discuss a full withdrawal, most of the troops were still in position, with reporters in the mountains overlooking Beirut seeing only scattered movement of military trucks heading toward the Bekaa Valley.
Two of Lebanon's neighbors, Israel and Jordan, have said they would accept nothing less than a complete Syrian withdrawal, reports CBS News Correspondent Robert Berger.
"We hope that Syria will make the right decisions," Jordan's King Abdullah said in an interview on Israeli television.
Israel believes the Syrian redeployment is a tactic to divert international pressure, while maintaining the occupation of Lebanon.
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, founded 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.
Participants 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?"
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 in Lebanon 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.