In a resolution, MEPs called for an "impartial and effective" investigation into the killing of 22-year-old Mahsa Amini.
Members of the European Parliament have called for EU sanctions against Iranian security forces over their "widespread, intentional and disproportionate" use of force against demonstrators.
"The response of the Iranian security and police forces to the protests has been violent, indiscriminate and unrestrained, and has resulted in substantial loss of life as well as a large number of injuries," MEPs said in a non-binding resolution approved on Thursday afternoon.
Separately, Josep Borrell, the EU's foreign policy chief, said the bloc was considering slapping restrictive measures on Iran, but without giving further details.
After holding a phone call with his Iranian counterpart, Hossein Amir-Abdollahian, the diplomat said the EU was contemplating "all options."
Borrell intends to discuss the issue during the next meeting of EU foreign affairs ministers, scheduled to take place on 17 October. Sanctions require the unanimity of the 27 member states.
"People in Iran, as anywhere else, have the right to peaceful protest. It's as simple as that," Borrell said earlier this week. "And it's clear that this right has not been ensured."
Iran has been swept by street protests since 22-year-old Mahsa Amini was killed under suspicious circumstances after being arrested by the country's morality police, which is tasked with enforcing a strict interpretation of sharia law.
Police claimed Amini was in breach of the state-imposed dress code because her hijab was too loose and exposed too much hair. She fell into a coma and died three days later in what witnesses have described as an act of police brutality.
Her death has sparked street protests around the country, with women taking off her headscarves and cutting their hair in open defiance of the country's moral rules.
Much of the popular discontent has been directed against Iran's supreme leader, Ayatollah Ali Khamenei, who is the country's highest political and religious authority. Khamenei has condemned the "rioting" as a foreign plot instigated by the US and Israel, without providing any evidence.
National security forces have launched a crackdown on protestors in a bid to quash the nationwide uprising, going as far as shutting down the Internet.
Iran Human Rights, a Norway-based organisation, estimated that at least 154 people, including children, have been killed in the repression.
On Thursday, MEPs expressed their solidarity with protesters, particularly young women, and asked for an "impartial and effective" investigation into Amini's death and the allegations of torture that surround her passing.
Lawmakers also called on Iranian authorities to release all those who have been arrested "for exercising their rights to freedom of expression, association and peaceful assembly," including journalists.
The EU should impose sanctions on the authorities involved in Amini's death and the crackdown of street protests, MEPs said.
The Islamic Republic of Iran is already under a heavy regime of Western sanctions, which are at the centre of negotiations to revive the nuclear deal.
Beyond the events of recent weeks, MEPs issued a wider denunciation of Iran's political system, touching upon the "systematic discrimination" against women and minority groups, the "degrading" law that makes veiling compulsory, the "severe" restrictions on women's sexual health, the "steady" deterioration of human rights and the increasing use of the death penalty inside the country.
The resolution was approved by a show of hands.
The vote was preceded by a debate on Tuesday, during which Swedish MEP Abir Al-Sahlani cut her hair in a show of solidarity with Iranian women.