About 150 people had gathered near the interior ministry, demanding the release of political prisoners.
It follows Tuesday’s “day of rage” when hundreds calling for democracy protested in Damascus and Aleppo.
The interior ministry said “infiltrators” tried to stir chaos. But among those held was a 10-year-old boy.
Activists say he was with his father – and both were detained.
Others included 69-year-old Damascus university philosophy professor and author Tayeb Tizini, and leading human rights activist Suhair Atassi.
“They pulled Suhair by her hair and took her away,” one demonstrator said, according to Reuters.
Journalist and activist Mazen Darwish said that he was set free only after being held for five hours in the military security branch’s detention centre alongside 20 others, including women. A further 15 were still being detained by state security, he said.
“When I showed them my international press card they shouted and said ‘Why were you standing among protesters and not among the journalists?’,” said Mr Darwish, head of the Syrian Centre for Freedom of Speech.
No further information about those detained was immediately available