CARACAS — Questions and condemnation of Venezuela’s leadership poured in on Tuesday following the suspicious death of an opposition activist who authorities said evaded justice by throwing himself from the 10th floor of a police building.Fernando Albán, a Caracas councilman, was arrested Friday at Caracas’s international airport upon arrival from a trip to New York to galvanize world opinion against President Nicolás Maduro.The country’s chief prosecutor said that Mr. Albán killed himself Monday by leaping from the state police agency’s headquarters, where he was being held for alleged involvement in a plot to kill Mr. Maduro using explosive-laden drones in August.But opposition leaders, backed by several foreign governments, have cast doubt on the official version, and accused Mr. Maduro’s government of torturing and murdering the politician.“There are so many different reports and quite a lot of speculation on exactly what happened: whether Mr. Albán committed suicide, whether he was thrown, or what exactly happened,” said Ravina Shamdasani of the United Nations’ human rights office in Geneva.While dozens of demonstrators were killed when security forces cracked down on protests last year, the death of a government opponent in state custody shocked many Venezuelans, who associate such deaths with the right-wing dictatorships that governed several countries in South America in the 1970s, and which Mr. Maduro frequently denounces.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 or ability to exercise their legal rights. The government denies they are political prisoners.The American State Department said it condemned Mr. Albán’s death, calling it a human rights violation, while Spain’s leftist government called for consultations with the Venezuelan ambassador in Madrid over the death. The United Nations urged officials in Caracas to undertake an impartial investigation to resolve the conflicting versions of how Mr. Alban died.As international criticism of Mr. Maduro’s handling of the case grew, Mr. Albán’s family, fellow politicians and family members of other jailed activists paid a final tribute to him Tuesday in a ceremony at the opposition-controlled Congress.A who’s who of opposition leaders took turns filing past Mr. Albán’s coffin, which was draped in the Venezuelan flag. His grieving sister brought a framed portrait of her deeply religious brother shadowed by an image of Jesus Christ.Hours after his death, a few dozen of Mr. Albán’s supporters gathered outside the police building yelling “Maduro killer!” and contending that Mr. Albán had been murdered. They later assembled outside Caracas’s morgue to demand his body be handed over as rumors spread he would be cremated.“There’s no doubt this was an assassination,” the opposition leader Julio Borges said in a video filmed from exile in neighboring Colombia, without providing evidence of his claim. “The only thing left for this government is torture, violence and destruction.”Mr. Borges, who led the delegation to the U.N., said Mr. 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 to kill Mr. Maduro using two drones loaded with explosives.More than two dozen people have been jailed on suspicion of involvement in the plot, which Mr. Maduro claims was orchestrated by Mr. Borges with the support of Colombia and the United States.The chief prosecutor Tarek William Saab ordered an investigation into the circumstances surrounding Mr. Albán’s death, which he classified as a suicide.In brief comments on state TV, he said Mr. Albán was in the headquarters of Venezuela’s intelligence police waiting to be transferred to a courthouse when he asked to use a bathroom. He then threw himself from the 10th floor of the building, officials said.Senator Bob Corker, the chairman of the Senate Foreign Relations Committee, who was in Venezuela for meetings with Mr. Maduro and his opponents, called Mr. Albán’s death while in the government’s custody “disturbing.”“The government has a responsibility to ensure all understand how that could have happened,” he said in a message on Twitter.Also expressing concern was Venezuela’s Catholic bishops’ conference and Luis Almagro, the head of the Organization of American States and a sharp critic of Venezuela’s government. He called Mr. Albán’s death “the direct responsibility of a torturing and homicidal regime.”Friends said the councilman was a family man and devout Catholic.“Albán is a very Christian person, with deep spiritual convictions that go contrary to a decision to take one’s life,” said Joel Garcia, a lawyer who met with Mr. Albán the night before in the tribunal, and said he seemed calm.