The confrontations at dozens of universities prompted the threat of a tougher crackdown in a seventh week of demonstrations sparked by the death of 22-year-old Mahsa Amini after she was arrested by the morality police for attire deemed inappropriate.
"Security is the red line of the Islamic Republic, and we will not allow the enemy to implement in any way its plans to undermine this valuable national asset," hardline President Ebrahim Raisi said on Sunday, according to state media.
Iranians from all walks of life have taken to the streets since Amini's death in protests that the clerical rulers said were endangering the Islamic Republic's security.
Authorities have accused Islamic Iran's arch-enemies the United States and Israel and their local agents of being behind the unrest to destabilise the country.
What began as outrage over Amini's death on September 16 has evolved into one of the toughest challenges to clerical rulers since the 1979 revolution, with some protesters calling for the death of Supreme Leader Ayatollah Ali Khamenei.
The top commander of Iran's Revolutionary Guards told protesters that Saturday would be their last day of taking to the streets, the harshest warning yet by Iranian authorities.
Nevertheless, videos on social media, unverifiable by Reuters, showed confrontations between students and riot police and Basij forces on Sunday at universities all over Iran.
Videos from universities showed Basij forces opening fire at students.
Across the country, security forces tried to block students inside university buildings, firing tear gas and beating protesters with sticks. The students, who appeared to be unarmed, pushed back, with some chanting "dishonoured Basij get lost" and "Death to Khamenei".
Social media reported arrests of at least a dozen doctors, journalists and artists since Saturday. The activist HRANA news agency said 283 protesters had been killed in the unrest as of Saturday including 44 minors. Some 34 members of the security forces were also killed.
More than 14,000 people have been arrested, including 253 students, in protests in 132 cities and towns, and 122 universities, it said.
The Guards and its affiliated Basij force have crushed dissent in the past. They said on Sunday, "seditionists" were insulting them at universities and in the streets, and warned they may use more force if the anti-government unrest continued.
"So far, Basijis have shown restraint and they have been patient," the head of the Revolutionary Guards in the Khorasan Junubi province, Brigadier General Mohammadreza Mahdavi, was quoted as saying by state news agency IRNA.
"But it will get out of our control if the situation continues."
