We condemn this arbitrary and illegal measure and ask that Riad Turk be freed immediately and that those responsible for his arrest be prosecuted

Opposition statement

Mr Turk, a former Communist leader, was released from prison in 1998 after serving 17 years for his opposition to late President Hafez al-Assad.

He was reportedly rearrested on Saturday at a doctor's clinic in the coastal city of Tartous, 300 kilometres (186 miles) north-west of Damascus, where was undergoing treatment for a heart ailment.

It is not yet clear why the he has been arrested.

Unjustified

"We condemn this arbitrary and illegal measure and ask that Riad Turk be freed immediately and that those responsible for his arrest be prosecuted," the statement said.

"Mr Turk has on several occasions called for dialogue and reconciliation and his arrest cannot be justified," it added.

The Paris-based Arab Commission for Human Rights (CADH) has also called for Mr Turk's release.

"The CADH holds the Syrian authorities responsible for what may happen to Mr Turk ... and considers that the decision to arrest him crosses red lines in relations with the democratic opposition," it said in a statement.

Openly critical

In the past Mr Turk has called for human rights reform in Syria and a lifting of the martial law that has been in effect since 1963.

In August, in his first major address since his release from jail, Mr Turk attacked the "hereditary" succession which allowed Bashar al-Assad to succeed to the presidency in July of last year after the death of his father, Hafez.

Hafez al-Assad had run the country with an iron fist for 30 years.

The new president did implement several reforms and released 600 political prisoners belonging to several banned political parties, but diplomats say that further reform is being blocked by the government's old guard.