The protest turned nasty on Saturday, when security forces tried to clear protesters who gathered outside the government headquarters in anticipation of a visit by David Welch, the US assistant secretary of state.
Some of the protesters, waving Lebanese flags and carrying placards protesting against US influence in Lebanon and the Middle East, pelted police with stones.
One placard read: "Welch is not welcome in Lebanon".
Aljazeera reported that police used tear gas to disperse the anti-US demonstrators near government offices.
The demonstrators mostly belonged to the Baath Party, the Syrian National Social Party, Hizb Allah and the Amal Movement.
Welch, who met with several Lebanese officials on Saturday, also held talks with Fouad Siniora, the Lebanese prime minister, at the government headquarters, where he re-issued a warning to Syria to fully and unconditionally co-operate with the UN investigation into the assassination of Rafiq al-Hariri, former Lebanese prime minister, or face "further action".
Lebanese protesters opposed
Welch's visit to their country
"If Syrian obstruction continues, we will not hesitate to refer this matter back to the (UN Security) Council for further action."
In Damascus, the official Sana news agency said Welch sought to increase "pressure on Syria because of its policy calling for stability and comprehensive peace, and opposition to aggression and occupation."
Aljazeera quoted a Syrian Information Ministry official as saying that Damascus considers the US diplomat's remarks as a ratcheting-up of pressure against Damascus with the aim of increasing tension between Syria and Lebanon.
Welch and Elliott Abrams, a US deputy national security adviser, began a trip to the region last week.
Their visit had been postponed because of the Israeli prime minister's stroke, but US has resumed efforts to resolve the Israeli-Palestinian conflict without Ariel Sharon.
The US is assailed in much of the Arab world for what many perceive as its pro-Israeli stance.
But Washington has pledged to support Lebanon since Syria pulled its forces out of its smaller neighbour in April amid intense international and local pressure following the killing of Rafiq al-Hariri, the former Lebanese prime minister.
A UN inquiry has already implicated Syrian officials in the murder. Damascus denies any role.