Amnesty International condemned the excessive use of force by Iranian security forces today that saw scores of protesters beaten and detained during student-led demonstrations.

Security forces - including the volunteer Basij militia - used batons and tear gas to disperse opposition supporters in the wake of threats by officials that all demonstrations would be considered illegal and met by force.

By the end of the day, the number of protestors arrested was not known.

Since the disputed election a pattern has emerged of the authorities preventing peaceful demonstrations, and then hastily resorting to violence against people who nevertheless choose to exercise their right to freedom of expression and assembly." said Hassiba Hadj Sahraoui, deputy director of Amnesty International's Middle East and North Africa program.

"All those arrested for simply attending today's demonstrations should be immediately and unconditionally released. The Iranian authorities continue to treat peaceful dissenters as criminals in violation of Iran's Constitution." 

Others arrested should be released unless they are to be charged with a recognizably criminal offence and tried promptly and fairly.

Thousands of opposition supporters and students had gathered in Tehran and cities across the country to mark the anniversary of the killing of three students by security forces in 1953. In recent years the anniversary has become a focus for demonstrations by students on campuses calling for reform and greater respect for human rights.

One eyewitness told Amnesty International that students from Shahid Beheshti University marched alongside the walls of Evin Prison in northern Tehran chanting "political prisoners should be free" and "students will die, we won't accept oppression." 

Another told Amnesty International that central Esfahan and the university in the southern part of that city was full of Basij militia and plain clothed security officers to clamp down on any protests.

Over the course of the day, Amnesty International has been receiving reports of confrontations between plain-clothed security officers believed to be Basij and students at sites throughout the country, such as at Mazandaran and Sari universities, in the north of the country. Since the morning, security forces in Sari are said to have told students not to attend university.

According to reports, police used plastic bullets at Amir Kabir University in Tehran to stop students inside the campus from joining up with protestors outside. 

In recent weeks, students suspected of organizing the protests had received threats and scores were detained in an attempt to stifle the dissent.

Protestors also faced other repressive restrictions as the authorities blocked the use of the internet and mobile phones. 

In a further crackdown the authorities banned foreign media from covering the protests.

On Saturday the security forces arrested up to 29 women taking part in a silent protest in Tehran. The group, Mourning Mothers, which is made up of mothers whose children died in the post-election violence and other women who gather every week to call for an end to the human rights violation which have taken place since the election, including justice for their dead children.