Palestinians demand Abbas resignation at Ramallah protest
Relatives of Nizar Banat, who died after security forces stormed his home and violently arrested him, were at the forefront of the protest in Ramallah where Abbas's Palestinian Authority (PA) is based.
Banat's mourning mother held up a portrait of her son.
Demonstrators also carried pictures of Banat, a vocal critic of alleged corruption within the PA, while others held up a huge banner that carried the words "Abbas Leave", an AFP reporter said.
"This rally is a message of allegiance to Nizar Banat and to the authorities, who must bring to justice those responsible for his death," Hassan Khreishah, a former head of the Palestinian Legislative Council, told AFP.
Palestinian police and security forces blocked the road leading to Abbas's headquarters in Ramallah.
A counter-protest organised by Abbas's Fatah faction was meanwhile held in the city of Hebron to express support for the Palestinian leader.
Banat's death on June 24 sparked days of protest in the occupied West Bank and triggered international condemnation.
The 43-year-old had registered as a candidate in Palestinian parliamentary elections that were set for May, until Abbas postponed them indefinitely.
According to an autopsy, he had been beaten on the head, chest, neck, legs and hands, with less than an hour elapsing between his arrest and his death.
On Monday, Banat's family said they would reject the conclusions of an official inquiry and instead called for an international investigation.
His death sparked global outrage. Protests decrying Banat's death have swept the West Bank and been violently suppressed by PA security forces and plainclothes officers.
UN Middle East peace envoy Tor Wennesland said the "perpetrators must be brought to justice", while the EU called for a "full, independent and transparent investigation".
The United States said it was "deeply disturbed" by the death, and urged a transparent investigation.
On Thursday, more than a dozen Palestinian security troops, including officers, have been detained over their alleged involvement in the death of prominent activist Nizar Banat, according to his cousin.
"We have been informed that 14 of those who took part in the assassination of Banat are being detained in Jericho," Ammar Banat told local media.
He added that while the family was given the news early on Thursday, they did not take it seriously. He called on the Palestinian Authority to release the names of those it had arrested.
"We will not allow there to be scapegoats who are made to bear the responsibility of those [actually] involved. All must be held to account for their crimes, actions, and words," the deceased's cousin said.
PA security forces spokesman Talal Dweikat confirmed the number arrested in relation to Banat's death, saying they had been referred to the judiciary pending "completion of the investigation procedures", WAFA reported.
The arrests follow a preliminary probe into Banat's death by a high-level PA committee, reportedly assembled by intelligence and military authorities.