Iranian police fired tear gas and live bullets as they fought back thousands of unarmed protesters on the streets of Tehran.
By Damien McElroy, Foreign Affairs Correspondent
There were bloody clashes as young people launched a fresh wave of anti-government protests on the country's official Students Day.
Police used warning shots, baton charges and gas but failed to stop rallies, sit-ins and campus marches across the capital.
Universities in several cities, including Tehran's top seats of learning, were sealed off as guards checked identity cards of people trying to join the student demonstrations.
Earlier in the day, the authorities detained 23 members of a protest group of grieving mothers. They included the mother of Neda Agha-Soltan, known as the "Angel of Freedom", who was shot by pro-government militia at the height of demonstrations against Mahmoud Ahmadinejad's re-election in June.
Hajar Rostami-Motlaq has enraged the authorities by condemning pro-government students who accused British agents of killing Miss Soltan.
She was later released but friends expressed concern for other members of the protest group, Mourning Mothers of Iran, who were rounded up at a weekly protest in Tehran's Laleh Park.
Supporters of opposition leader Mirhossein Mousavi chanted "Death to the dictator" and "Do not be scared. We are all together", according to witnesses at the rallies on university campuses.
The authorities had deployed troops and militias in anticipation of the protests, the fourth such outbreak under the opposition strategy of using official public holidays as cover for protests.
Television pictures showed hundreds of men and women gathering in front of Tehran university gates, pulling at the padlocked fence and making hand gestures of "V" for victory.
Images of the crowds taken on mobile phones were sent to the West despite attempts to suppress news of the demonstrations by confining the staff of foreign news organisations indoors.
"Security forces are beating demonstrators, men and women. Some of them are injured and bleeding," said one Tehran eyewitness.
"There's anxiety that there will be violence and shooting. I shout slogans and demonstrate but try not to provoke any clash with the security," said Kouhyar Goudarzi, a student. "We are worried."
Iran's Revolutionary Guards and Basij militia had warned the opposition not to use the rally to revive protests against the clerical establishment after the June vote.
Internet and mobile phone connections were also affected by an official clampdown. "The network in central Tehran and near Tehran university is completely down," said one website.
Mr Mousavi issued a message that warned the country's leadership that popular frustration was continuing to grow.
"You fight people on the streets, but you are constantly losing your dignity in people's minds," Mr Mousavi, a former prime minister, said. "Even if you silence all the universities, what are you going to do with the society?"
Mr Ahmadinejad has retained the backing of Iran's Supreme Leader, Ayatollah Ali Khamenei, despite deep splits in Iran's clerical establishment. Hashemi Rafsanjani, the former president, broke weeks of silence to apologise for the hardliners refusal to relent in the face of widespread popular dissatisfaction.
Iranian students were commemorating three scholars who were killed by Shah Mohamed Reza Pahlavi's security forces on Dec 7, 1953, as they protested the sacking of nationalist prime minister Mohammed Mossadegh.