Government forces sought to reimpose an iron grip on Tehran and other cities yesterday, a day after Iran was convulsed by violence which left at least eight dead and the Islamic regime facing a crisis of authority.

Plainclothes agents and special police units were reported to be deployed in overwhelming numbers in four of Tehran's main squares – Enghelab, Haft-e Tir, Valiasr and Ferdowsi – which formed part of the focal point of Sunday's fierce confrontations. Three city-centre underground stations were also closed as authorities sought to block off gathering points for protesters.

But skirmishes were reported between opposition supporters and government forces in Haft-e Tir square. Teargas was said to have been fired at opposition supporters who had gathered outside Ebn-e Sina hospital, where the nephew of the reformist leader Mir Hossein Mousavi was pronounced dead after having being shot on Sunday.

The clampdown followed the arrests of at least 10 senior political figures overnight, including three Mousavi aides and Ebrahim Yazdi, 78, a former foreign minister and one-time adviser to the late Ayatollah Ruhollah Khomeini, spiritual leader of the 1979 Islamic revolution.

Last night Barack Obama praised "the courage and the conviction of the Iranian people" while condemning the government for attacking demonstrators with "the iron fist of brutality". The US president said Iran's troubles were caused by its leaders' decision "to govern through fear and tyranny" and demanded "the immediate release of all who have been unjustly detained within Iran".

The crackdown reprised the response to June's post-election protests, when many prominent activists were detained after mass demonstrations against President Mahmoud Ahmadinejad's disputed re-election. But they served only to emphasise Iran's devastated political landscape as opposition figures voiced outrage that Sunday's Ashura ceremony, a day of reverence for the Shia Islam martyr Imam Hossein, became steeped in blood after security forces allegedly opened fire on demonstrators.

While the government continued to deny that accusation yesterday, witnesses reported that plainclothes Basij militia members had hit protesters' heads with steel batons in an apparent attempt to inflict fatal injuries. Family and friends of many of the injured avoided taking them to hospitals, where security agents were waiting to identify and arrest casualties. Doctors were instead called to people's homes to administer emergency treatment.

The fatalities raised questions about the religious legitimacy of Iran's supreme leader, Ayatollah Ali Khamenei, who stood accused of breaking the basic tenets of Islam by permitting killing during the holy month of Muharram. Another reformist leader, Mehdi Karroubi, implied that Khamenei was worse than the former shah, whose troops never opened fire on Ashura. "What has happened to this religious system that it orders the killing of innocent people during the holy day of Ashura?" Karroubi said.

His comments were echoed by Mohsen Makhmalbaf, the Paris-based Iranian film-maker and unofficial spokesman for Mousavi. He compared Khamenei to the 7th century Umayyad caliph Yazid, hated in Shia Islam as the slayer of Imam Hossein, and added: "I'm upset with myself for fighting against the shah. At least when he realised people didn't want him, he left the country."

Makhmalbaf's statement, posted on his website, went further by comparing Khamenei to Saddam Hussein, the former Iraqi leader who is hated in Iran for invading the country in 1980. "Khamenei … we are going to pull you up from your Saddam-like well and shed light on your face – but not with the flashlight of an American soldier," the statement said.

Makhmalbaf also alleged that Ali Mousavi Khamane, the nephew of the reformist leader, had been targeted by state security agents who went to his house to assassinate him.

Suspicions multiplied when the dead man's relatives accused the security forces of removing his body from Ebn-e Sina hospital, purportedly for a forensic investigation. Opposition figures suggested the move was intended to prevent the Mousavi family from holding mourning ceremonies, which the government fears could turn into even bigger protests.

Sunday's violence spread far beyond Tehran. Disturbances were reported in other cities, including Isfahan, Mashhad, Shiraz and Tabriz, where a further four people were reportedly killed.

More ominously for the government's authority, clashes were also said to have occurred in smaller cities such as Babol, Arak and Orumiyeh.

Sunday's scenes evoked memories of the last days of the shah, when demonstrators took to the streets to confront the former monarch's forces. However, Hossein Bastani, an Iranian political analyst, warned against drawing parallels with the present situation. "The shah's regime collapsed when many members of the army, who were just doing their military service and were really supporters of Khomeini, rebelled and went over to the other side," he said. "This regime has learned from that. They have invested too much in creating their own dedicated forces. They may be a tiny proportion of Iran's population, but they are absolutely committed to the Islamic republic, very violent and very efficient."