University students have staged sit-down strikes in support of the anti-government protests in Iran, despite an intensifying crackdown by authorities.

Videos and photographs posted online showed young men and women gathered in corridors, courtyards and roads at campuses in Tehran and other cities.

Some held placards warning there would be no classes until fellow students arrested while protesting were freed.

Activists say 300 have been detained since the unrest erupted six weeks ago.

The protests were sparked by the death in custody of a young woman who was accused by morality police of wearing her hijab "improperly".

They have since evolved into one of the most serious challenges to the country's clerical establishment since the 1979 Islamic Revolution.

On Tuesday, Iran's Human Rights Activists News Agency (HRANA) posted what it said were pictures of sit-in strikes by students at several university campuses in the capital, including Sharif University of Technology, the female-only Al-Zahra University, and Amir Kabir University of Technology.

A video of the sit-in at Shahid Beheshti University showed a group chanting and holding a placard saying that the sit-in was "for imprisoned students".

Videos published on social media shows students are protesting at #Beheshti University despite violent crackdown. Students of several faculties staged a sit-in, demanding the release of their class mates.#Iran #MahsaAmini https://t.co/L8ESim5WQV

— Rana Rahimpour (@ranarahimpour) November 1, 2022

Similar sit-ins took place at universities in the central cities of Yazd and Isfahan, and in the southern city of Shiraz.

In the north-eastern city of Sanandaj, female students at Kurdistan University of Medical Sciences were meanwhile filmed waving their headscarves in the air to chants of "Woman, Life, Freedom" - a signature slogan of the protests.

As the strikes took place, Telegram channel Students United reported the arrests of two students at Amir Kabir University on Tuesday. HRANA also said three had been arrested on Monday at Islamic Azad University of Shiraz and a fourth in the south-western city of Zahedan.

"Universities should be protected and be a safe space for students," tweeted Azar Mansouri, a women's rights activist and head of the pro-reform Union of Islamic Iran People Party.

Protests have also continued unabated outside universities in defiance of violent clampdowns by security forces.

Opposition activist collective 1500tasvir posted videos that it said showed security forces firing sound grenades and issuing threats via loudhailers, in an attempt to stop residents of blocks of flats in Tehran's Ekbatan district shouting anti-establishment slogans from their windows at night.

Right now: the Islamic Republic’s forces of oppression have staged an attack on residents of Ekbatan, Tehran. The residents report having heard and felt explosion sounds.#MahsaAmini

— 1500tasvir_en (@1500tasvir_en) October 31, 2022

In one video, a security officer could apparently be heard saying: "We would even cut off the heads of our own wives and children if necessary."

On Monday, HRANA reported that at least 287 people, including 46 children, have been killed by security forces since the protests began. Another 14,160 protesters have been detained, according to the news agency.

A state-run newspaper reported that 35 security personnel had been killed by what authorities have portrayed as foreign-backed "rioters".

The chief prosecutor in Tehran announced on Monday that preparations were under way for the public mass trials of 1,000 people charged with "acts of sabotage", including the murder of security guards and arson.