Iranian police have fired tear gas at opposition supporters participating in a rally in the centre of the capital, Tehran, called in solidarity with the popular uprisings in Egypt and Tunisia.
A BBC producer, who was affected by the gas, said there had been severe clashes and described a scene of "total chaos".
There were also reports of protests in the cities of Isfahan and Shiraz.
Earlier, the police placed opposition leader Mir Hossein Mousavi under house arrest, according to his website.
It said the move was intended to prevent the former prime minister attending the march in Tehran, which the authorities had prohibited. The road leading to Mr Mousavi's house was also blocked by police vans.
Fellow opposition leader Mehdi Karroubi a former Speaker of parliament and senior cleric, is also reportedly under de facto house arrest.
Both men disputed the re-election of President Mahmoud Ahmadinejad in June 2009, which triggered mass protests that drew the largest crowds in Iran since the Islamic Revolution in 1979 and led to a brutal crackdown.
The opposition says more than 80 of its supporters were killed over the next six months, a figure the government disputes. Several have been sentenced to death, and dozens jailed.
Internet blocked
In their first major show of dissent since December 2009, thousands of opposition supporters defied the government ban and gathered at Tehran's Azadi Square on Monday. They chanted "Death to dictators".
Riot police and plain-clothes police backed by the Republican Guards used tear gas to disperse the protesters, he adds.
Other witnesses said police also fired paintball guns at demonstrators.
The fiercest clashes were reported on Azerbaijan Street, close to Azadi Square, and one witness said a number of ambulances had come and gone.
Mr Mousavi's website, Kaleme.com, said that according to unconfirmed reports, "hundreds of protesters" had been arrested. There has been no official confirmation.
Police also later surrounded Tehran University and Sharif University, and the houses of former President Mohammad Khatami and Abdollah Nuri, a former interior minister and head of Tehran City Council.
Earlier, an activist wearing a green headband - the colour of the main opposition movement - was detained after he climbed a tall crane in the capital and began inviting people to attend Monday's demonstration.
The BBC's James Reynolds: This is a 'second go' for the opposition movement who tried to force a revolution in 2009.
Although Iran's establishment officially supports the Egyptian protests, it says the rallies in Iran are a "political move" by opposition leaders.
In anticipation of the rally, the authorities stepped up security in the capital, blocked access to internet sites, and started jamming satellite news channels. Police helicopters also hovered overhead.
Analysts say Tehran is trying to stop opposition groups from using the Egypt rally as a means to re-ignite anti-government protests of 2009.
Both the Iranian government and the opposition have claimed credit for the recent popular uprisings in Tunisia and Egypt.
The government says the mass protests were inspired by Iran's 1979 revolution, while the opposition says its 2009 protests encouraged the unrest. The opposition also says peaceful rallies do not need permission.
Last week, White House national security spokesman Tommy Vietor said: "For all of its empty talk about Egypt, the government of Iran should allow the Iranian people the same universal right to peacefully assemble and demonstrate in Tehran that the people are exercising in Cairo."
Turkish President Abdullah Gul, who is on a visit to Iran, meanwhile warned on Monday that "when leaders and heads of countries do not pay attention to the demands of their nations, the people themselves take action to achieve their demands".
source : bbc.co.uk
