Hundreds of thousands of people have gathered in Lebanon's capital, Beirut, to applaud Syria's role in the country and reject Western "interference".
Many Lebanese believe they share Syria's interests
"We are here to thank Syria which has stayed by our side for many years," the head of the Syrian-allied Hezbollah group told cheering supporters.
The crowd dwarfed previous opposition protests urging Syrian troops to leave.
In a speech in Washington, President George W Bush reiterated US demands for complete Syrian withdrawal.
The Beirut rally was organised by Hezbollah, a powerful political and military organisation of Shia Muslims, the largest religious minority in Lebanon.
Hezbollah officials handed out Lebanese flags and directed the men and women to separate sections, but the crowds were so large they spilled out of Riad al-Solh Square into surrounding streets.
The group's leader, Hassan Nasrallah, said there would be further demonstrations in other Lebanese towns including Tripoli in the north, Nabatiya in the south and in the eastern Bekaa valley.
He also warned the US military not to interfere in Lebanon, saying: "If the American fleet lands in Lebanon, it will be defeated."
The rally took place just a few hundred metres from Martyrs Square, where predominantly Christian, Druze and some Sunni Muslims, have been holding their anti-Syria protests.
A line of military vehicles separated the two squares to avoid any possible friction between the rival demonstrators.
Hezbollah has grown into a powerful political force as it conducted a military campaign to push Israeli forces out of south Lebanon, which happened in 2000.
It has several MPs in parliament, an influential television station and a network of welfare and charitable organisations.
Its demonstration of support came as Syria - facing intense international pressure - began moving some of its 14,000 troops in Lebanon east, as part of a pull-back plan adopted by Lebanese and Syrian leaders in Damascus on Monday.
SYRIA IN LEBANON
Military intervention begins in 1976
30,000 troops in Lebanon during 1980s, currently 14,000
Syrian forces help end Lebanese civil war in 1990 and maintain peace
Calls for Syrian withdrawal increase in 2000 after Israeli pullout from southern Lebanon
UN resolution calling for foreign forces' withdrawal in Sept 2004
The US has described the plan as a "half-measure" and said all Syrian troops and intelligence officers should leave Lebanon immediately in line with UN resolution 1559, sponsored jointly by Washington and Paris.
Speaking at Washington's National Defense University, President Bush did not mention the latest rally, but repeated US demands that Syria end its presence in Lebanon before elections take place in May.
"Syria... has a long history of supporting terrorist groups determined to sow division and chaos in the Middle East. And there's every possibility they will try this strategy again," he said.
Syrian troops arrived in neighbouring Lebanon as peacekeepers during the 1975-1990 civil war, since when Damascus has kept a firm hold on the political and financial spheres in Lebanon.
France, Germany and Russia have also called for a Syrian withdrawal, as has its long-standing Arab ally, Saudi Arabia.
On Monday, large crowds gathered in Martyrs Square for protests to mark three weeks since the death of former Prime Minister Rafik Hariri, killed in a car bomb blamed by many on Syria. Damascus has consistently denied responsibility.