A US diplomat observing the demonstration was also briefly detained, prompting a protest from the US government and an apology from the Syrians, according to State Department spokesman Richard Boucher.
The activists were released after a few hours of interrogation, but their detentions highlighted tensions between the Syrian government and prodemocracy reformists.
"Breaking up a peaceful protest and arresting people for expressing their opinion is a mistake that the authorities bear responsibility for," said Hassan Abdul-Azim, who heads the independent Syrian National Democratic Gathering, a prodemocracy group.
Had it been allowed to continue, the protest outside Parliament -- organized by the Committees for the Defense of Democratic Liberties and Human Rights in Syria -- would have been the first of its kind in a country where political activity is tightly controlled.
The head of the group, Aktham Naisse, told the Associated Press that activists had expected to be arrested and that authorities had pressured him to cancel the planned sit-in because it "served American interests at this time."
When the group of around 20 protesters arrived at the Parliament building in downtown Damascus, Syrian riot police and plainclothes security agents were waiting.
The police told the activists to disperse, but one man raised a banner that read: "Freedom for Prisoners of Opinion and Conscience."
Agents quickly tore the banner up and snatched the notebooks of journalists gathered to cover the protest.
At one point, Naisse, a lawyer from the northern town of Latakia who had been imprisoned for seven years for political activity, told the activists to raise their hands in the air. "We're going to prison; we are not afraid," he declared.
A spokesman at the US Embassy in Damascus said the diplomat who was detained was held at a police facility for about an hour before being released. The diplomat was not identified.
Syrian security officials later apologized for "what clearly was a breach of diplomatic practice," the spokesman said, adding that the US Embassy had protested the incident "in the strongest terms."
It was unclear how many activists were arrested. The Human Rights Association in Syria said in a statement that 30 protesters were beaten up and arrested. Others said the number was less.
But Naisse, after his release, told AP that 104 people were detained.
Since the US-led war on Iraq, Syria has been under pressure from the US administration to reform.
