"A police major accompanied with two policemen arrested Kamal al-Labwani at the Damascus (International) Airport upon his return from the United States," Ammar Qurabi, a prominent human rights activist, said on Tuesday.
"I was there ... I asked them where are you taking him? They said to the interrogation branch" at the Interior Ministry, he said. Officials were not immediately available for comment.
Al-Labwani met senior US administration officials in Washington earlier this month after he left Brussels where he met some European Union officials, Qurabi said.
His visit to the US came in for criticism from several opposition figures, including prominent activist Hasan Abd al-Azim, who distanced his leftist party from the trip.
Abd al-Azim has often said that he refuses to seek foreign intervention to achieve political reform in the Arab state.
Many see recent Syrian steps as
as a bid to deflect US pressure
Qurabi urged the authorities to release al-Labwani. "We hope this does not represent a step backward after the release of 190 political prisoners a few days ago. I hope he will be released soon."
President Bashar al-Assad pardoned 190 political prisoners on 2 November in a step widely seen as aiming at broader internal reforms to deflect US pressure on Syria after charges that it was linked to the killing of a former Lebanese prime minister. Syria denies any involvement.
In 2004, al-Labwani and a handful of other activists formed the Liberal Democratic Union.
His group suffered a blow earlier this year after a dispute erupted between al-Labwani and its co-founders, most of whom abandoned the body.