"The Iranian regime's inhumanity knows no bounds," German Foreign Minister Annalena Baerbock said.
France and Germany have strongly condemned Iran's first execution of a protester involved in nationwide anti-government demonstrations.
Mohsen Shekari was accused of blocking a street and attacking a member of the security forces in Tehran, according to Iranian state media. He was hanged on Thursday morning.
"The Iranian regime's inhumanity knows no bounds," German Foreign Minister Annalena Baerbock said on Twitter. Paris also signalled its "strongest condemnation".
"#MoshenShekari was sentenced and executed in a perfidious summary procedure because he disagreed with the regime," Baerbock said.
"But the threat of execution will not stifle people's desire for freedom," she added.
For France, "this execution comes on top of other serious and unacceptable violations," foreign ministry spokeswoman Anne-Claire Legendre told a news briefing.
"We recall our firmest attachment to the right to peaceful protest," she added, stressing that the aspirations of the demonstrators "are legitimate and must be heard".
Thursday's execution in Iran is thought to be the first since the country's protests began. The demonstrations were triggered by the death in September of Mahsa Amini, a 22-year-old Iranian Kurdish woman who died after being arrested by police for breaking strict Islamic dress codes.
Shekari was arrested on September 25, then convicted on November 20 on the charge of "moharebeh," a Farsi word meaning “waging war against God”, Iran's Mizan news agency reported.
Amnesty International has said Iranian authorities are seeking the death penalty for at least 21 people in what it called "sham trials designed to intimidate those participating in the popular uprising".
The human rights group has called on Tehran to quash all death sentences, refrain from seeking capital sentences, and drop all charges against peaceful protesters.
At least 475 people have been killed in the demonstrations amid a heavy-handed security crackdown, according to Human Rights Activists in Iran, a group that has been monitoring the protests. Over 18,000 have been detained by authorities.