Nizar Banat's health apparently deteriorated during the operation by Palestinian security forces at his residence in Dura and he was subsequently pronounced dead at a hospital in Hebron, the city's governor Jibrin al-Bakri said.

Demonstrators in Ramallah held up banners of Banat's face and some demanded the resignation of Palestinian President Mahmoud Abbas.

The circumstances of Banat's death are under investigation.

His relatives accused the police of brutally abusing him during the arrest, according to media reports.

Banat had wanted to run in parliamentary elections scheduled for May - the first to be held in 15 years - but Abbas postponed them in April, blaming a dispute with Israel over voting rights in Jerusalem.

Abbas has not named a new date.

Banat had repeatedly accused the Palestinian Authority of grave human rights violations and corruption.

He had been arrested several times in the past.

Since the cancellation of the election, there have been reports of tougher action by the Palestinian Authority against its critics.

Opposing Palestinian factions were united in their condemnation of Banat's death.

Ghassan Jadallah, the leader of the Democratic Reform bloc within Abbas' Fatah movement, described it as a "murder".

"The assassination of Banat reflects the bloody policy of the authority in settling scores," Hamas leader Sami Abu Zuhri said.

Banat's death was part of a "policy of silencing, persecution, arrest and killing," the Popular Front for the Liberation of Palestine said.

Australian Associated Press