The demonstrators blocked traffic as they gathered in a central square to shout pro-Syrian and anti-U.S. slogans.
The peaceful rally took place just some 300 meters from where supporters of Lebanese opposition parties have been holding daily rallies to demand a complete Syrian withdrawal.
The latest demonstration ratchets up tensions in Lebanon that have been running high since a bomb attack in Beirut killed former Lebanese Prime Minister Rafiq Hariri on 14 February.
Opposition demonstrators, mainly Christian and Druze with some Sunnis, have accused Damascus of killing Hariri to keep him from increasing pressure on Syria. Damascus and the pro-Syrian Lebanese government have denied the charges.
Until today, Hizballah -- which backs the government -- had stayed off the streets. But its leaders issued frequent warnings that there could be civil strife if Syrian troops quit the country completely. They have also accused the opposition parties of seeking to replace Syrian influence with that of the United States and Israel.
The Hizballah party has close ties with Damascus, which along with Iran, is one of its main financial backers.
Now, Hizballah's taking to the streets marks a potentially volatile development in the crisis. The party, which includes a large militia, is the only Lebanese faction that remains heavily armed from Lebanon's 1975-1990 civil war.
The Hizballah was founded in 1982 by Iran's Revolutionary Guard to combat Israel's invasion of Lebanon the same year. The group later became internationally notorious for kidnapping Westerners in Lebanon in the 1980s, for killing more than 200 U.S. soldiers in a suicide bombing of their Beirut headquarters, and hijacking a U.S. commercial airliner in 1985.
But the group gained widespread popularity in Lebanon for waging a guerrilla war against Israeli troops maintaining a security belt in the south of Lebanon. Its attacks contributed to Israel's decision to withdraw from southern Lebanon in 2000.
The militant Shi'a group's move to the forefront of the current crisis also sets the stage for a potential new contest between Hizballah and the U.S. and other Western states pushing for a full Syrian withdrawal.
Washington, Britain, France, and Germany have made it clear they will maintain their pressure on Damascus.
U.S. State Department spokesman Richard Boucher put the U.S. position this way in remarks on 7 March. "Syria must leave Lebanon and leave Lebanon now," Boucher said. "The time for words and communiques and statements and discussions and more meetings is really gone. We need to see Syrian action."
As the West continues to press Damascus, the Hizballah could emerge as a major player in any Syrian efforts to resist. Whether the party chooses to play that role peacefully or violently is a major question to be answered in the days ahead.
Damascus promised on 7 March to pull back its roughly 14,000 troops in Lebanon to the eastern Bekaa Valley as part of a two-stage withdrawal process. But it set no date for a final pullback of the troops into Syria itself.
Syria has maintained a force in Lebanon since 1976, when it intervened in Lebanon's civil war.
The civil war lasted 15 years, until a 1990 peace accord established a fragile balance between the country's diverse religious groups.