Both the Syrian Human Rights Organisation and Anwar Bunni, a human-rights lawyer, said Ali Abdullah, who spent five months behind bars last year, had been arrested at his home for "unknown reasons".

"The pursuit of this campaign of political arrests will cause more suffering," the group said, calling on the government to free all political detainees.

Abdullah was last arrested in May 2005 for reading a message by a Muslim Brotherhood leader during a political discussion at the Atassi salon, a venue devoted to reform in Syria.

He was released in November along with 190 political prisoners as part of "overall reforms" in Syria, official media reported at the time.

Abdullah's son, Omar, was arrested at the weekend along with a second student, Diab Seerieh, for wanting to "form a democratic gathering of youths to discuss young people's problems," Bunni said.

The rights group also said that Mohammed Najati Tayyara, vice-president of the Human Rights Association in Syria (HRAS), was arrested on Wednesday for unknown reasons.

Bunni added that Tayyara had attended a meeting of Syrian opposition members held in the US last month.

HRAS said Mohammed Walid al-Kabir al-Hosni, a 65-year-old father of four, had also been arrested on Tuesday "after he had been sought by security forces for several months".

He was arrested "probably for having spoken his political opinions in public places," the group said.

Bunni also called on the authorities to "end this campaign aiming to terrorise activists because it only increases the list of human rights violations".