Protesters called for Taysir Alluni's release, saying his health is deteriorating. They urged the Spanish authorities to provide him better medical care.
Alluni's wife has said that his health is deteriorating because of prison conditions, where he is held in isolation for 20 hours a day. He is permitted four hours outside his cell, she says, but he is not allowed to talk to anyone.
The International Federation of Journalists (IFJ) has asked member organisations in Spain to investigate Alluni's prison conditions.
The correspondent has been held in solitary confinement since his detention two months ago following his re-arrest on charges of links to al-Qaida.
"This case has caused anxiety to journalists, not least because after months on bail with no apparent problems, he [Alluni] was taken back into custody"
International Federation of Journalists
Alluni, Syrian by birth but holding a Spanish passport, was first arrested in 2003 on suspicion of links with al-Qaida as part of an investigation into Islamist operations in Spain.
As a correspondent for Aljazeera, he became well known for his work in Afghanistan during the US-led invasion.
But two months after being taken into custody, he was released on bail. Judges ruled he should be freed pending a trial because of his poor heart condition.
The indictment alleges he used his role as a reporter to take cash and messages to al-Qaida members.
But Alluni says the evidence is based on badly translated telephone conversations. He was taken back into custody in November 2004.
The IFJ general-secretary, Aidan White, said: "This case has caused anxiety to journalists, not least because after months on bail with no apparent problems, he was taken back into custody.
"Now we have reports that he is in deteriorating health. It is important that he is treated in a humane and compassionate manner."
Lawyers have asked the Spanish authorities to give Alluni a full medical examination by heart and back specialists, but they say there has been no response.