Riad Seif has been arrested several times for dissident activity

He was detained for attending a meeting of pro-democratic reformers, they say.

It brings to 11 the number opposition activists arrested since the meeting December, reports say.

Syria does not usually comment on such arrests. Human rights groups say Syria holds hundreds of political prisoners, some without charge or trial.

Mr Seif, who suffers from prostate cancer, had already been banned from leaving Syria for medical treatment, a restriction he criticised as a "slow death sentence".

The US state department condemned the arrest and called for the immediate release of everyone detained "simply for their desire to speak their minds and engage in a political dialogue in Syria".

Deputy spokesman Tom Casey said the US had been urging Syria to allow Mr Seif to travel abroad for treatment.

Symbol of reform

Mr Seif heads the secretariat of the Damascus Declaration group, which was formed in 2005 and which last met in December to form a national council, bringing together 163 opposition members.

The National Organisation for Human Rights in Syria (NOHRS) said Mr Seif was taken into custody on Monday evening by the security authorities after a warrant was issued against him for participating in the meeting.

It listed the charges subsequently laid against him as "harming the image of the state", "stoking ethnic and sectarian division", "disseminating false information" and "belonging to a secret organisation seeking to alter the social and economic base of the state".

An NOHRS statement described him as "one of the symbols of the Damascus Spring", the period of political dialogue after the death of former President Hafez al-Assad in 2000.

His successor, his son Bashar, initially tolerated the discussion groups which came into being, but a tough clampdown began in 2001.

Mr Seif has spent time in jail for criticising the younger President Assad's government.