The wife of acclaimed Syrian artist Youssef Abdelke, whose works are displayed in museums around the world, appealed for his release on Saturday, after his arrest by regime security forces.
He had been arrested at a security checkpoint in the western port city of Tartus two days previously, along with two fellow members of the Communist Labour Party.
Abdelke, born in 1951, is also a member of the internal opposition National Coordination Body for Democratic Change (NCB).
He has been jailed in the past by the Syrian authorities.
Abdelke's friends and family have rallied around his wife Hala Alabdalla, a prominent film-maker, and launched a campaign on Facebook calling for his release from jail.
"His detention may be a small thing when compared to the bloody violence and destruction (gripping Syria) but it's still unacceptable," Alabdalla told AFP in a telephone interview.
Hours before his detention, Abdelke signed a petition written by artists expressing their commitment to "the principles of the popular revolution launched in March 2011... aimed at establishing a democratic, pluralist political system" in Syria.
Novelists, poets and musicians also signed the petition which, she said, demands the ouster of President Bashar al-Assad and the transfer of power to an interim government, under UN supervision.
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"Anyone who works peacefully and from inside Syria for the revolution (against Assad) to succeed... is sacred and shouldn't be touched," Alabdalla said.
In a message posted on the Facebook page calling for his release, she wrote: "Freedom for Youssef Abdelke, freedom for all Syria."
Alabdalla said she lost touch with her husband, a Christian, late on Thursday as he arrived at a security checkpoint on the outskirts of Tartus.
With him were Tawfiq Omran and Adnan al-Dibs, both members of the communist party and the NCB, said the opposition group which supports the uprising but rejects foreign intervention in the conflict now in its third year.
Abdelke has spent "half his life in regime's jails and in forced exile", said a statement on the Facebook page set up to demand his release.
He was jailed in the past over his membership of the communist party and in 1981 moved to Paris -- where he had studied fine arts -- until his return to Syria 25 years later.
One of Syria's best known artists, Abdelke has had his paintings on display at the British Museum, in the Institut du Monde Arabe in Paris and in other galleries around the world.
Alabdalla says that her husband, who was born in the northeastern city of Qamishli, has been banned from travelling since the uprising began.
Rights groups say tens of thousands of people are being held in the Syrian regime's jails.