A Palestinian activist who died in the custody of President Mahmoud Abbas's security forces on Thursday suffered blows to the head, a human rights group said after a post-mortem.
The family of Palestinian Authority critic Nizar Banat said security forces broke into his house in the occupied West Bank city of Hebron on Thursday night and beat him with metal bars before arresting him.
A Palestinian Authority representative could not be reached for comment on a post-mortem commissioned by the Independent Commission for Human Rights (ICHR).
The authority also declined to comment on the circumstances of Banat's death.
Earlier, hundreds of people marched on Mr Abbas's residence in Ramallah to demand he resign over Banat's death, which drew calls from the UN, the US and the EU for a transparent investigation.
Prime Minister Mohammad Shtayyeh, appointed by Mr Abbas, ordered the creation of a committee into Banat's death that would include the ICHR.
But in a sign of public distrust with the Palestinian Authority inquiry, ICHR said it would lead its own investigation.
ICHR conducted the post-mortem with the family's permission, and announced its initial findings on Thursday night.
It said the examination "showed injuries represented by bruises and abrasions in many areas of the body, including the head, neck, shoulders, chest, back, and upper and lower extremities, with binding marks on the wrists and rib fractures".
"The preliminary autopsy results also indicate ... an unnatural death, but determining the principal cause of death, from a clinical point of view, requires waiting for laboratory results from tissue samples," it said.
Palestinians renewed protests in central Ramallah later on Thursday. Some clashed with security forces, who threw tear gas and stun grenades to disperse them.
Banat, 43, was a social activist who had accused Mr Abbas's administration of corruption. This included questions about a short-lived coronavirus vaccine exchange with Israel this month and Mr Abbas's postponement of a long-delayed election in May.
Banat registered as a parliamentary candidate for that contest.
Mr Abbas regularly arrests his critics, human rights groups claimed, and has ruled the authority by decree for well over a decade.
"The crushing death of activist Nizar Al Banat shortly after his arrest by the PA is no anomaly. PA security forces have for years systematically [and] arbitrarily arrested, mistreated and tortured critics and dissidents," said Omar Shakir of Human Rights Watch.
The Palestinian Authority rejects accusations it is corrupt and that it arrests people for their political views. It also denies torture.
Hebron Governor Jibreen Al Bakri said in a statement that Banat's arrest was on the orders of the Palestinian Authority attorney general.
Banat's family told Reuters that security forces broke into their house in the middle of the night, pulled Banat from his bed and started to beat him.
"They hit him on his head with iron bars, which they had used to open the windows," Banat's cousin, Hussein, 21, said. "They beat him for eight minutes. If you came to arrest him, take him. Why the brutality?"
The US State Department said Washington was deeply disturbed by Banat's death and called on the Palestinian Authority to conduct a thorough and transparent investigation.
"We have serious concerns about Palestinian Authority restrictions on the exercise of freedom of expression by Palestinians and harassment of civil society activists and organisations," State Department spokesman Ned Price said.
The UN Middle East peace envoy, Tor Wennesland, said on Twitter he was alarmed and saddened by the death, and also called for a transparent investigation.
The EU delegation to the Palestinians said on Twitter it was shocked and saddened, and called for a "full, independent and transparent investigation should be conducted immediately".
The Palestinian Authority exercises limited self-rule in the Israeli-occupied West Bank, home to 3.1 million Palestinians.
On Monday, Banat, who had 100,000 Facebook followers, condemned Palestinian Authority officials as mercenaries for the coronavirus vaccine exchange deal, which the authority cancelled.
Banat planned to run for the Palestinian parliament on May 22. Mr Abbas called off the election, blaming Israeli controls over Palestinians in East Jerusalem. Opponents said Mr Abbas cancelled the vote to avoid losing, which he denies.