Iranian pro-government supporters march during a rally in support of the regime after authorities declared the end of deadly unrest, in the city of Mashhad Thursday. (Nima Najafzadeh/AFP/Getty Images)

More than a thousand people have been rounded up and detained in Iran over the past week, rights groups and the State Department said Thursday, as authorities try to quell the largest street protests in nearly a decade. 

Amnesty International warned that those being held risk torture and ill-treatment in the country’s prisons, calling for the release of those arrested for demonstrating peacefully. Earlier this week, the head of Iran’s Revolutionary Court warned that arrested demonstrators could face the death penalty. 

“The Iranian authorities have an appalling track record of carrying out mass arbitrary arrests of peaceful demonstrators,” said Philip Luther, the regional research and advocacy director at Amnesty. “Given the alarming scale of the current wave of arrests, it is highly likely that many of those held are peaceful protesters who have been detained arbitrarily and now find themselves in prisons where conditions are dire and torture is a common tool to extract confessions and punish dissidents.”

An unverified video circulating online claimed to show families of detainees waiting outside Tehran’s notorious Evin prison. There, at least 423 detainees were registered in just two days over New Year’s, Amnesty said, citing the Human Rights Activist News Agency. The site, which focuses on Iranian rights abuses, said the number of detentions across the country was likely “much higher” than the 1,000 reported.  

Hundreds are being held in Evin prison’s “quarantine section,” Amnesty said, a screening area for new arrivals that has a capacity for only 180. 

The demonstrations, fueled by economic grievances, unemployment and corruption, erupted a week ago in the northeastern city of Mashhad, fanning out across the country. The authorities banned popular messaging and social media apps, restricted cellphone networks and slowed Internet speeds in an attempt to prevent demonstrations from being organized or publicized. 

Iran’s army chief Maj. Gen. Abdolrahim Mousavi said Thursday that the demonstrations were so small that the police had managed to stamp them out and his troops remained on standby if needed, Iranian media said.

Reports of anti-government protests and unverified videos of demonstrations continued to trickle out Thursday. A number of large pro-government rallies were also held.

Holly Dagres, an Iran analyst and curator of the Iranist newsletter, said the crackdown by security forces hadn’t been as brutal as in 2009, when Iranians held huge demonstrations to protest the results of national elections. This time, protests have been smaller but more widely dispersed across provincial cities and towns.  

“The Iranian government has mostly relied on slowing the Internet and mobile phone services to stop the protests, and have done a good job as evident by the lower turnout,” she said. “That isn’t to say there hasn’t been a crackdown.” 

Iran has tried to blame the unrest, which has claimed at least 22 lives, on the meddling of outside powers, including the United States, and one Iranian official on Thursday accused the CIA of being an instigator.

The State Department on Thursday said it supports the “legitimate aspirations of the Iranian people” and condemned the mass arrests in the “strongest possible terms.”

“We have ample authorities to hold accountable those who commit violence against protestors, contribute to censorship, or steal from the people of Iran,” the statement said. “To the regime’s victims, we say: You will not be forgotten.”