Clashes between pro-reformists and security forces in Tehran have left several people injured, with one person reported killed.
Thousands of anti-government protesters marched on Monday on Enghelab and Azadi streets [which connect and create a straight path through the city centre], with a heavy presence in Enghelab Square and Vali-Asr Street.
Quoting witnesses, the AP news wire reported that at least three protesters injured by bullets were taken to a hospital in central Tehran, while dozens more were hospitalised because of severe wounds as a result of being beaten.
The semi-official Fars news agency said one person had been shot dead and several wounded by protesters.
"One person was shot dead and several were wounded by seditionists (opposition supporters) who staged a rally in Tehran," Fars said, without giving further details.
Al Jazeera's Dorsa Jabbari, reporting from Tehran, confirmed reports that security forces used tear gas, pepper spray and batons against the protesters.
As with other foreign media, she was prohibited by government order to witness the demonstrations.
Jabbari said that she had received reports that up to 10,000 security personnel had been deployed to prevent protesters from gathering at Azadi Square, where the marches, originating from various points in Tehran, were expected to converge.
The AFP news agency reported that police fired paintball bullets on protesters.
One video, posted on Youtube (claiming to be from Monday's protests) shows people chanting, "political prisoners must be freed." A woman then cries that tear gas has been deployed, dispersing the crowd.
On the Facebook page used to organise the marches, there were also reports of shooting in or around Enghelab Square, as well as demonstrations in the cities of Mashhad, Shiraz and Kermanshah.
Cashes between police and demonstrators - resulting in dozens of arrests - took place in Isfahan, the country's third largest city.
Twitter and Facebook posts said Mir Hossein Mousavi, the opposition leader and former presidential candidate, and his wife, Zahrah Rahnavard, had joined one of the marches.
Mehdi Karroubi, the other prominent opposition leader, is still under house arrest.
Al Jazeera was unable to confirm whether Mousavi and Rahnavard had joined the protest, and at last report, Kaleme.com, a pro-reformist website, said that security forces had prevented the couple from leaving their home.
As night fell in Iran, the BBC reported that city lights were being turned off and that security forces were attacking protesters in the dark.
While many of the protesters reflected on the day's marches on Twitter and Facebook, Youtube videos show that hundreds were still on the streets after dark, setting fire to rubbish bins and barricades, chanting anti-government slogans.
Monday's marches were organised as a one-day event and it is unclear if further protests will take place overnight or tomorrow.
A message on posted by the organisers of the demonstrations posted on the 25 Bahman Facebook site - the site's title reflecting today's date on the Iranian calendar - seemed to indicate that there might be more protests.
"The 25 Bahman group will try to announce the programme for of protests for tonight and tomorrow shortly," it read.
"Please stand by via any means of communication you have. We are victorious."
The current security clampdown is reminiscent of the one that crushed a wave of protests after the disputed re-election of Mahmoud Ahmadinejad, the Iranian president, in June 2009.
Opposition supporters revived a tactic from the 2009 protests, shouting "Allahu Akbar" or God is Great, and "Death to the dictator", from rooftops and balconies on Monday in a sign of defiance towards Iran's leadership.
Several opposition activists and aides to Mousavi and Karroubi have been arrested in recent days.
Hillary Clinton, the US secretary of state, hailed the "courage" of the protesters, and pressed Tehran to follow Egypt's example and "open up" its political system.
Our correspondent in the capital said that as far as Iran's leaders are concerned, Monday's protests "are not a reflection of what people actually want".
They believe these are small groups of individuals who have ulterior motives, they are a threat to national security and therefore the security forces are necessary to prevent them from becoming a threat inside the country," said Jabbari.
Amnesty International, the London-based human rights group, criticised Iranian authorities for opposing Monday's protests and making dozens of arrests, saying the crackdown was aimed at blocking the work of activists and stifling dissent.
"Iranians have a right to gather to peacefully express their support for the people of Egypt and Tunisia," said Hassiba Hadj-Sahraoui, Amnesty International's Middle East and North Africa deputy director.
"While the authorities have a responsibility to maintain public order, this should be no excuse to ban and disperse protests by those who choose to exercise that right."
There was no mention of Monday's demonstrations on state-run television stations or websites.
Instead, one station replayed interviews it did with those who attended the march celebrating the 32nd anniversary of the Islamic revolution on Friday.