Dozens of students have been detained by Iranian authorities who have arrested more than 1,000 people in their attempt to quell anti-government protests, according to Iranian MPs.

Mahmoud Sadeghi, a reformist MP, was quoted by the Iranian labour news agency as saying that around 90 students have been detained with at least 10 unaccounted for.

The scale of the crackdown emerged as the shadow foreign secretary, Emily Thornberry, defended Jeremy Corbyn over his silence on the unrest in Iran, saying the Labour party takes an approach of “extreme caution” when it comes to the politics of the Islamic republic.

Thornberry told the BBC it was impossible to determine what political forces lay behind the protests, which began on 28 December and are said to have led to at least 21 deaths – mostly of protesters but also some security guards, according to officials.

Corbyn has been under increasing pressure to speak out about the protests. Tom Tugendhat, who chairs the Commons foreign affairs committee, said it was “extraordinary” that Labour had not spoken out against the Iranian regime.

The international community is split over how to respond to the protests. A US attempt to raise the issue at a UN security council discussion on Friday was opposed by Russia. When the discussion took place, France rejected American efforts to make the protests an excuse for ditching the 2015 nuclear deal.

Iran has tried to recast the protests against the clerical establishment as the result of foreign interference. Thousands of government supporters staged rallies on Saturday, with state television showing protesters in cities including Amol, Semnan and Shadegan waving Iranian flags and chanting “Death to America”, “Death to Israel” and “Death to Britain”.

The provincial governor in north-eastern Mashhad, where the protests started, was quoted as saying that 85% of detainees there had been released after signing a pledge not to take to the streets again.

Tehran University has set up a committee to track the fate of arrested students. Sadeghi, who represents a Tehran district, said: “It seems that the total number of detainees is around 90. Ten students from universities in Tehran and some other cities are in an uncertain position, and … it is still unknown which body has detained them.”

According to Reuters, Iran has several parallel security bodies and arrests are often not immediately announced. Videos that have appeared on social media in recent days showed relatives of detainees gathering outside prisons in search of their loved ones.

Thornberry spoke to the BBC’s Political Thinking podcast to explain her reluctance to make a statement, and followed that with a lengthy social media post detailing her position.

“Our approach now is one of extreme caution when it comes to Iran and a recognition that the society in Iran is an immensely complex one, and seemingly contradictory,” she said.

“For example, with these current riots, sometimes they are calling to reinstate the monarchy, sometimes they are calling out against [Ayatollah Ali] Khamenei, sometimes they are calling for Khamenei, sometimes they are calling for the price of eggs.”