NEW YORK, April 24 (Reuters) - U.S.-based Human Rights Watch demanded on Tuesday that the president of Tunisia immediately release a political activist who it said has been illegally convicted multiple times for the same offense.

Human Rights Watch said Daniel Zarrouk was imprisoned in 1992, and since his arrest has been been convicted four times for his membership in the Islamist Nahda movement, which is banned in the north African state.

In a letter to Tunisian President Zine al-Abidine Ben Ali, Human Rights Watch also called for a review of all cases of prisoners serving multiple sentences for a single offense, which is said violated international and domestic law.

"Tunisia's judiciary should respond substantively to appeals from prisoners who claim they were unjustly convicted more than once for the same offense," said Sarah Leah Whitson, Human Rights Watch Middle East and North Africa executive director. "And Daniel Zarrouk -- whose case is flagrantly unfair -- should be freed immediately."

Human Rights Watch said Tunisian courts had imposed a total sentence of more than 20 years in prison for his membership of Nahda and other charges. It understood Zarrouk's sentences had been commuted and that he was due for release in 2009.

"Repeat convictions for membership of a banned group are cynically used to lengthen prison terms for Zarrouk and, most likely, other dissidents," Whitson said.

In November rights groups said Tunisia freed 55 political prisoners under an annual amnesty to mark the anniversary of the president's rise to power, but Amnesty International said at least 100 Nahda prisoners who were also sentenced after unfair trials in the early 1990s had not been released.

The Tunisian government has said it is committed to furthering democracy and respect for human rights, insisting critics of its rights record are biased and mirror the views of a minority bent on smearing the country's image abroad.

The country flatly denies torture and says it has no political prisoners.

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