The families of men incarcerated in Tehran's notorious Evin prison protested Tuesday near the Iranian president's office, complaining of mistreatment of their loved ones, activists said.
The gathering came after reports of violence in the jail during an inspection of inmates at Section 350, which houses political prisoners.
Several prisoners were beaten and hurt in Thursday's incident, according to Kaleme, an opposition website, but Iran's prisons chief said it was "totally wrong" to say guards had committed assault.
In Tuesday's protest, filmed by one participant and posted on Kaleme.com, dozens of demonstrators gathered in a street near the presidential compound.
In the short video, they chanted: "Evin has become Palestine, government why are you sitting and doing nothing?" referring to alleged Israeli violence against Palestinians.
The families, Kaleme said, want to meet government officials to discuss the alleged violence at Evin and they have also demanded a response from President Hassan Rouhani.
Rouhani is a self-declared moderate who promised greater accountability from government after taking office in August.
Amnesty International has expressed concern about the Evin incident, reporting that 100 prison guards dressed in riot gear entered Section 350, purportedly to conduct a search.
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The rights watchdog said at least four severely injured prisoners were transferred to an outside hospital, while another 26 inmates were hurt.
Iranian authorities have denied any wrongdoing, with Justice Minister Mostafa Pourmohammadi saying "only one or two prisoners suffered minor injuries" after resisting a routine inspection.
In the evening, state television aired footage it said was taken on the day of the incident, showing prisoners at one cell block sitting on the ground in protest.
They appeared to be refusing to leave prior to the inspection, prompting two guards to pick up one prisoner and remove him.
The footage then cut to prisoners out in the yard throwing objects at guards through a window, which had been partly shattered.
MP Hossein Sobhani Nia of the national security committee said Monday that during a subsequent search of the section "15 people had refused to allow inspection, items such as a cell phone, a SIM card and even alcohol were confiscated".
He said electronic devices found in Section 350 had been used to spread "false information to foreign and anti-revolutionary media."
But nine lawmakers demanded Pourmohammadi "pursue the case," after they dismissed as unsatisfactory accounts provided by prison officials, the ISNA news agency reported.
Other lawmakers have requested video footage from the prison, media reports said.