The journalists issued a statement on Monday denouncing the arrest of Alluni as "illegal and unethical".
The statement said it was "illegal because no evidence whatsoever has been produced against him, and unethical because hounding a journalist for his views or ideology flies in the face of human rights and democratic ethics".
The statement urged the Spanish authorities to immediately free Alluni and apologise to his family for his arrest and detention.
Alluni, who holds Spanish citizenship, was arrested 10 days days ago in southern Spain on suspicion of having links to al-Qaida.
A few days ago, a Spanish judged decided to keep him in detention indefinitely.
The decision drew angry reactions from journalists around the world.
Aljazeera has demanded Alluni's immediate release and has accused the Spanish of acting on false information from Israel and the United States.