The United Nations has said it will investigate the death in custody of an opposition councillor jailed by Venezuela’s intelligence police on allegations that he plotted to kill President Nicolás Maduro.

Venezuelan officials said that Fernando Albán killed himself by leaping from the 10th floor of the state police agency’s headquarters. But opposition leaders denied the official version.

Ravina Shamdasani with the Office of United Nations High Commissioner for Human Rights said Venezuelan officials were obligated to ensure Albán’s safety while in their custody. The investigation into his death will be part of the UN’s wider investigation of alleged human rights abuses, she said.

“We are concerned about news of his death,” Shamdasani said. “We do indeed call for a transparent investigation to clarify the circumstances.”

Albán, 56, was taken into custody on Friday at Caracas’s international airport upon arriving from New York, according to his lawyer. He was in the US accompanying other members of his First Justice party for meetings with foreign dignitaries attending the United Nations general assembly.

A few dozen of Albán’s supporters gathered outside the police building yelling “Maduro killer!” contending that he had been murdered.

“There’s no doubt this was an assassination,” the opposition leader Julio Borges said in a video from exile in neighbouring Colombia, without providing evidence of his claim. “The only thing left for this government is torture, violence and destruction.”

While Venezuelans last year saw dozens of youths killed in violent street battles with security forces, the death of activists or government opponents while in state custody is a fate more associated with the far deadlier, rightwing dictatorships that dominated much of South America in the 1970s.

The opposition claims that more than 100 Venezuelans opposed to Maduro are being held as “political prisoners”, some for more than four years, with little access to the outside world and their legal rights routinely trampled on. The government denies they are political prisoners.

Borges, who led the delegation to the UN, said Albán’s wife told him that her husband had been under intense pressure to testify against him in the ongoing investigation into the alleged plot in early August to kill Maduro using two drones loaded with explosives.

More than two dozen people have been jailed on suspicion of involvement in the plot, which Maduro claims was orchestrated by Borges with the support of Colombia and the US.

The chief prosecutor, Tarek William Saab, ordered an investigation into the circumstances surrounding Albán’s death, which he classified as a suicide.

ISpeaking on state TV, he said Albán was in the waiting room of the headquarters of Venezuela’s intelligence police waiting to be transferred to a courthouse when he asked to use the bathroom. He then threw himself from the 10th floor of the building, officials said.

Borges, who said Albán was a personal friend, said the councillor – who represented a district in the Caracas area – was a family man and devout Catholic who would never kill himself.

Albán’s lawyer, Joel García, said his client seemed calm when they met on the night before in the tribunal. García said authorities cannot determine that a death is a suicide without an investigation and he would ask to be present at the autopsy.