Milad Zohrevand, 21, was put to death on Thursday morning at a prison in the western city of Hamadan, they said.
There was no confirmation from Iran's judiciary, which convicted Zohrevand of murdering a member of the Revolutionary Guards during a demonstration.
The sources said he was denied a lawyer throughout his detention and trial.
If confirmed, Zohrevand would be the eighth protester to be executed after last year's rallies.
A recent report by UN Secretary General António Guterres warned that in the seven previous cases, information "indicated that the judicial proceedings did not fulfil the requirements for due process and a fair trial".
It added: "Access to adequate and timely legal representation was frequently denied, with reports of coerced confessions, which may have been obtained as a result of torture."
The protests against Iran's clerical establishment erupted in September 2022 following the death in custody of Mahsa Amini, a 22-year-old woman who was detained by morality police for allegedly wearing her hijab "improperly".
Hundreds of people have been killed and thousands detained in a violent crackdown by security forces, which have portrayed the protests as "riots".
Two informed sources told BBC Persian that Milad Zohrevand was executed at Hamadan Central Prison at dawn on Thursday. But prison authorities had not handed over his body to his family by the afternoon, they said.
Hengaw, which focuses on Iran's Kurdish ethnic minority, said in a statement that Zohrevand had also not been notified that his execution was imminent or granted a final meeting with his family.
It strongly condemned the execution, which it said "not only violates the right to life but also egregiously infringes upon the human rights of the detainee and his family".
Hengaw also alleged that the Revolutionary Guards - a major military, political and economic force in Iran - had put pressure on authorities to carry out the execution.
It said this had happened without the explicit consent of the family of the officer whom Zohrevand was alleged to have killed.
Last week, Hamadan's prosecutor said a branch of the Supreme Court had confirmed the death sentence handed to a defendant convicted of the murder of Ali Nazari, a member of the Revolutionary Guards' intelligence service.
They did not identify the defendant, but legal collective Dadban cited sources as saying that it was Zohrevand. It also reported that his family had been pressured into not publicising his case.
Zohrevand was reportedly accused by prosecutors of being one of a group of five masked men who shot Nazari as he confronted students demonstrating at the Malayer Faculty of Medical Sciences in October 2022.
Iran is second only to China in the number of executions carried out annually.
The UN secretary general's report said Iran was carrying out executions "at an alarming rate", putting to death at least 419 people in the first seven months of the year. That is a 30% increase compared to the same period in 2022.
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