Anwar al-Bunni, a campaigner for political freedoms, was dragged from his Barzeh home on Wednesday by the security forces, his family said.
"Anwar asked them to show him an arrest warrant so he would go with them, but they forced him into the car, and drove away while he was shouting," his brother Akram al-Bunni said.
Nine people have been detained by the authorities since Sunday, human rights groups said.
Al-Bunni's detention came after he publicly condemned Sunday's arrest of another rights activist, Michael Kilo.
Bunni, Kilo and hundreds of other activists had signed a petition last week calling on the government to mend its ties with Lebanon.
Relations between Syria and Lebanon were badly damaged after last year's assassination of Rafiq al-Hariri, the former Lebanese prime minister, with many Lebanese blaming Syria for the killing.
A pro-government newspaper, Tishrin, condemned the petition, saying it was full of lies.
Despite calls for Kilo's release, the government has charged him with undermining the state, weakening national sentiment and instigating sectarian and racial divisions, according to rights activists.
The National Organisation for Human Rights in Syria said the number of arrests was the largest since a similar move in December 2001 when dozens of campaigners were detained.
Arab Press Freedom Watch - a group that campaigns for freedom of speech - condemned the crackdown and urged the world to put pressure on the Syrian government to release detainees.
"The Baath regime seems to have lost patience for the increasing voices of the opposition and decided to step up political detention," Freedom Watch said in a statement.