Al-Arabi is prominent among Mr Mubarak's critics
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The Egyptian journalists' syndicate has expressed "anger, horror and disgust" at the beating of a leading opposition journalist on Tuesday.
Abdul Halim Qandil said he was bundled into a car by four armed men on his way home from a Ramadan meal in Cairo.
Mr Qandil's al-Arabi newspaper has been outspoken against Egyptian President Hosni Mubarak's regime.
He said his attackers warned him to "stop talking" about important people in Egypt.
Scores of local journalists and opposition politicians have joined the protest against the attack.
Mr Qandil said he suspected Egyptian security agents, crime groups or shady businessmen may have been behind the attack.
"I can't accuse anybody, but I'm wondering if it might have something to do with the articles I write," Mr Qandil told AFP.
"The first word they uttered was 'so that you stop talking about the big ones'," he said
He said he was then gagged and blindfolded, beaten up and stripped before being dumped on the main motorway between Cairo and Suez.
Gamal Fahmi, of the Journalists' Syndicate, said the attack threatened to turn the political debate "into a violent dialogue that resorts to the ways of gangsters."
An interior minister official declined to comment, saying the incident was under investigation.