Fernando Alban was arrested last week for alleged involvement in the explosion of two drones during a military parade that was led by President Nicolas Maduro which injured seven soldiers in August.
Venezuela's government said on Monday that a jailed opposition lawmaker killed himself by jumping from the 10th floor of the state intelligence agency headquarters (SEBIN) where he was being held, but his party said he was murdered by the government.
Municipal lawmaker Fernando Alban, 56, was jailed last Friday for alleged involvement in the explosion of two drones during a military parade in August that was led by President Nicolas Maduro, Interior Minister Nestor Reverol said.
"At the moment he was going to be transported to court, while he was in the SEBIN waiting room, he jumped from the window of the building and fell, causing his death," Reverol wrote in a post on Twitter.
That account differed from the one by Chief Prosecutor Tarek Saab, who said in televised statements that Alban had asked to use a bathroom and jumped from there.
The opposition First Justice party, for which Alban was a Caracas municipal counselor, said he was murdered.
"With great pain and thirst for justice we tell the people of Venezuela ... that Councilman Fernando Alban was murdered at the hands of the regime of Nicolas Maduro," the party said.
First Justice's national coordinator, Julio Borges, said Alban's lifeless body was thrown from the SEBIN's headquarters on Caracas' Plaza Venezuela. He did not provide any evidence and did he say what led him to believe that Alban was killed.
The party said that Alban's arrest was owing to statements he made at recent United Nations meetings in New York City denouncing humans rights violations in Venezuela.
The government had previously given no reason for his arrest. On Monday, Reverol said Alban had also been investigated for "destabilising activities directed from abroad."
The chairman of the US Senate Foreign Relations Committee, Senator Bob Corker, called on Maduro to explain the reasons for Alban's death. Corker was in Caracas on Monday to meet with government officials and the opposition to assess Venezuela's economic crisis.
"This is disturbing and the government has a responsibility to ensure all understand how that could have happened," Corker, a Republican, said on Twitter.
The United States imposed new sanctions on several of Maduro's top allies last week as President Donald Trump urged UN members to support a "restoration of democracy" in Venezuela.
Alban's assistant, Mileidy Blanco, said she had begged him to leave Venezuela with his family.
"And he didn't do it. He loved the country too much," she said in an interview outside the SEBIN's headquarters, known as "The Tomb" for its underground detention facility.
The opposition and rights groups have said that Maduro's administration is holding hundreds of political prisoners on trumped-up charges intended to stifle dissent. The United Nations has said detainees are often subjected to ill treatment, including some documented cases that amount to torture.
Authorities have arrested at least 30 people linked to the drone explosions on August 2, which injured seven soldiers.
The government has denied that it is detaining people as political prisoners and has said that they were fairly jailed.
Alban's lawyer, Joel Garcia, told reporters that when he last saw Alban on Sunday "he was not beaten, he was fine." He said Alban told him that he had been interrogated but not tortured.
The Organization of American States (OAS) condemned Alban's death.
"Direct responsibility of a regime that tortures and murders. This criminal dictatorship must go now from Venezuela," OAS Secretary General Luis Almagro said on Twitter.