Scores of protesters descended on a two-story building used by the powerful Islamic Revolutionary Guards Corps (IRGC) in western Iran.
As demonstrators chanted antiestablishment slogans and threw stones at the building on January 3, IRGC members climbed on the roof and opened fire on the crowd.
That's according to Mohammad Heydari Ilami, a resident of Malekshahi, Ilam Province, who witnessed the deadly incident. His testimony has been collaborated by human rights groups.
"Those who didn't see them on the roof thought it was an air strike," said Ilami, who spoke to RFE/RL's Radio Farda. "But when some of the people who were shot fell to the ground, people realized they were shooting directly at them."
"A number of innocent people, mostly young people, were killed," Ilami added. "The number of wounded was high."
The Norway-based Hengaw rights group said five protesters were killed and dozens wounded. Iran Human Rights, an NGO also based in Norway, gave an identical death toll.
Iranian state media said security forces clashed with "rioters" who attempted to storm a police station, with "two assailants" killed.
The violence marked one of the deadliest single incidents since ongoing nationwide protests erupted on December 28. Iranians in dozens of cities and towns have taken to the streets to voice their anger over the state of the reeling economy, with the national currency in a free fall.
Radio Farda has so far verified the deaths of 20 protesters. Hengaw put the death toll at 25, including four children. The rights group said over 1,000 people have been arrested. HRANA, a network of rights activists, said at least 29 have been killed, including two security personnel.
The Islamic republic has witnessed several bouts of nationwide protests in recent years.
Demonstrators have called for greater social freedoms and political rights and voiced anger over soaring inflation and water scarcity. The authorities have typically responded with brute force.
Western Iran, including Ilam, has witnessed some of the deadliest government crackdowns during the latest round of protests.
'They Beat Everyone'
Just a day after the killing of the protesters in Malekshahi, the authorities raided the Imam Khomeini Hospital in the nearby city of Ilam where the wounded protesters were taken for treatment.
Members of the IRGC and special forces on January 4 surrounded the hospital, where a large crowd had gathered, including people seeking information about loved ones.
Ilami, the protester, said he witnessed security forces firing tear gas and smashing windows to get inside the hospital.
"Despite attacks by the security forces, including the use of tear gas, this crowd and the medical staff did not allow them to take the bodies and arrest the wounded," he said.
Eventually, security forces stormed the hospital, beating and arresting medical workers. It was unclear if injured protesters and their family members were also arrested.
"They beat everyone they saw, whether they were members of the public or medical staff," said Ilami. "They beat everyone."
In a statement issued on January 6, Amnesty International said the "Iranian security forces' attack on a hospital in Ilam, where injured protesters are seeking medical care or shelter, violates international law and exposes yet again how far the Iranian authorities are willing to go to crush dissent."
"Iran's security forces must immediately stop the unlawful use of force and firearms against protesters, end arbitrary arrests of those seeking treatment in hospital, ensure those injured receive the medical care they need, and respect the sanctity of medical facilities," added the international human rights group.
