Palestinian journalists have staged rallies for Alan Johnston

Mr Johnston was kidnapped after he left his Gaza office on 12 March and has not been heard from since.

A little-known militant group has claimed to have killed him, but the BBC says it cannot verify this.

In a statement from prison in Israel, Mr Barghouti called Mr Johnston a "friend of the Palestinian people".

Barghouti is one of the most prominent Palestinians held in Israel. He is serving five life terms for attacks which killed four Israelis and a Greek monk.

International protests

"From my cell, and in the name of 10,000 prisoners in the occupation jails, I appeal and call immediately for the release of journalist Alan Johnston, the friend of the Palestinian people," Mr Barghouti said.

He said kidnappings and attacks on civilians harmed the Palestinian cause.

On Sunday, the previously unknown Tawhid and Jihad (Holy War and Unity) brigades claimed it had killed Mr Johnston.

The corporation's Director General Mark Thompson said the BBC was still seeking clarification of the reporter's condition.

Mr Johnston, 44, has served as the BBC's Gaza correspondent for three years.

His posting was due to have finished in late March.

There have been international protests over his abduction and tens of thousands of messages of support have poured in for Mr Johnston.