TEHRAN – Iranian riot police fired tear gas and paintballs at protesters holding anti-government demonstrations in Tehran on Monday, websites and witnesses said, drawing a stern rebuke from Washington.

Police moved in when crowds of opposition supporters gathered in the capital's Azadi (Freedom) Square began chanting "Death to the Dictator!" -- a slogan used against President Mahmoud Ahmadinejad after disputed official results from a 2009 presidential election gave him a second term.

The website of opposition leader Mir Hossein Mousavi, kaleme.com, said that according to "unconfirmed reports, hundreds of protesters were arrested in Tehran."

There was no immediate official confirmation.

US Secretary of State Hillary Clinton hailed the "courage" and "aspirations" of the protesters and called on Iran to follow Egypt's example and "open up."

"We wish the opposition and the brave people in the streets across cities in Iran the same opportunity that they saw their Egyptian counterparts seize in the last week," Clinton told reporters during a visit to the US Congress.

"We support the universal rights of the Iranian people. They deserve to have the same rights" as those demanded by Egyptian protesters whose 18 straight days of mass demonstrations prompted longtime US ally Hosni Mubarak to step down, she said.

The anti-government demonstrations, held despite a ban on rallies, were the first in Tehran since February 11, 2010, when activists took to the streets to mark the 31st anniversary of the Islamic revolution.

Opposition website Rahesabz.net said clashes were reported near Tehran University and on the road connecting Azadi Square with Enghelab Square.

It said police fired tear gas as protesters chanted "Ya Hossein, Mir Hossein" a slogan from 2009 in support of Mousavi.

Rahesabz.net also reported chanting against supreme leader Ayatollah Ali Khamenei, with shouts of "Ben Ali Mubarak, It's your turn Syed Ali!"

"Police also barged into buses stuck in traffic on the road (between Azadi Square and Enghelab Square) and beat women passengers to spread fear among passengers," Kaleme.com reported.

It said protesters using roadside telephone booths and recording footage on mobiles were also subject to police assaults.

Websites and witnesses said thousands of opposition supporters had taken to the streets of Tehran in support of Arab revolts despite a heavy police deployment.

Some set fire to rubbish bins while chanting slogans in apparent reference to Ahmadinejad.

Mobile phone services were cut and there were power blackouts in areas where the protests were taking place, witnesses said.

The authorities earlier surrounded the house of Mousavi to prevent him from attending the rally which regime-backers said was a cover for protests similar to those which shook the foundations of the Islamic republic in 2009.

Fellow opposition leader Mehdi Karroubi has been under de facto house arrest, according to his website Sahamnews.org, while Rahesabz.net reported that former reformist president turned opposition backer Mohammad Khatami's house was also cordoned off.

While Iran backed the Arab uprisings in Tunisia and Egypt, the interior ministry banned the Monday rally planned by Mousavi and Karroubi.

Witnesses and websites said the opposition supporters had initially walked in scattered crowds silently to Azadi Square from across the capital as policemen kept a sharp watch.

Motorbike-borne riot police armed with shotguns, tear gas, batons, paintball guns and fire extinguishers were deployed in key squares to prevent the gatherings.

"Some policemen are chasing protesters in order to disperse them," a witness said, describing the scene at Imam Hussein Square where he said there were around 1,000 riot police.

More police and Basij militiamen were in Haft-e Tir square, a regular site for anti-government protests in 2009.

British Foreign Secretary William Hague called on Iran to allow its people to demonstrate freely.

"President Ahmadinejad last Friday told the Egyptian people that they had the right to express their own views about their country," Hague said.

"I call on the Iranian authorities to allow their own people the same right and to ensure that the security authorities exercise restraint."

Foreign media have been banned from on-the-spot reporting of the gatherings. Later on Monday, state television website reported that the demonstrations had ended and that crowds of people also demonstrated in support of the regime.

Mousavi and Karroubi and their supporters remain steadfast in rejecting Ahmadinejad's presidency, saying the hardliner was re-elected because of massive vote rigging in June 2009.

Their protests in the immediate aftermath of the election brought hundreds of thousands of people on to the streets of Tehran and other cities, shaking the regime and dividing the nation's elite clergy.

Iranian authorities crushed those demonstrations in a draconian crackdown in which scores of people were killed and wounded, and thousands arrested.