The Syrian authorities are expected to release the final members of a leading human rights group who were recently rounded up.
Police may have been reinforcing the ban on the group gathering
There was no explanation for the detentions on Sunday, the eve of the International Day for Human Rights.
About 30 members of the Damascus Declaration for Democratic Change were detained after a meeting they held in the Syrian capital on Saturday.
Most activists were released a few hours after the arrest.
Public gatherings by the group have been banned, and Sunday's arrest appeared to be an attempt by the Syrian authorities to enforce that rule.
The detention follows a meeting by 168 human rights activists on Saturday, at which they called for gradual and peaceful democratic change in Syria.
A leading member of the group told the BBC after his release that the move was a message to weaken the Syrian opposition movement.
When President Bashar al-Assad came to power in 2000, he raised hopes for democratic reform by releasing hundreds of political prisoners.
Repression soon followed however, seriously limiting the freedom of opposition in Syria.