The demonstrators -- opponents of the Syrian-backed government -- are demanding an inquiry into the assassination two weeks ago of former Prime Minister Rafik Hariri. They blame Syria for Hariri's death -- an allegation that Syria denies.
They are also calling for Syria to withdraw the around 15,000 troops it has stationed in Lebanon from the time of the country's civil war.
Demonstrators chanted "independence, freedom and sovereignty" and "Syria out, Syria out."
the protest was mostly peaceful and police and the army made no moves to prevent the gathering.
Most of the protesters appeared to be in their 20s, although some older people could be seen.
Daniella Sfair said it was her first time attending a demonstration, but she thought it was necessary to send a signal. "I am here because I want to say 'no' to what is happening in Lebanon," Sfair said. "The Syrians should be out. I don't like the manipulation of Syria of our government. We need some relation with them because we are neighbor, of course. But we don't need that [intervention]. Enough is enough."
The demonstration coincided with an expected confidence vote in the Lebanese parliament. Opposition legislators say their aim is to bring down the pro-Syrian government of Prime Minister Omar Karami.
Meanwhile, U.S. deputy assistant secretary of state David Satterfield was in Beirut on 28 February to renew U.S. calls for a Syrian troop withdrawal.
Syrian troops were used to stabilize Lebanon during the country's civil war in the 1980s. But many say the time has come for the troops to leave. Calls for a withdrawal have grown louder since Hakiri's death.
Syrian Foreign Minister Farouq Al-Shara, responding to the increased pressure, said on 27 February that Syria intends to carry out a preliminary withdrawal in accordance with a 1989 accord called the Taef agreement. "Syria is committed to implementing the Taef agreement, this agreement that has Arab and international support," Al-Shara said. "But for Syrian and Lebanese reasons, the implementation of parts of the Taef agreement was delayed."
That withdrawal would see Syrian forces moved to Lebanon's eastern Bekaa Valley.