Police in Tehran today attacked hundreds of demonstrators with teargas and fired weapons into the air after the protesters defied a warning from the Revolutionary Guard that it would crush any further protests.

As helicopters hovered overhead, hundreds of police moved to disperse about 200 protesters gathered at Haft-e-Tir Square, the scene of huge demonstrations last week.

Witnesses said police prevented any groups, even small ones, from gathering. At the underground station by the square, police prevented people from standing still, asking them to keep walking and separating those who were walking together.

The new clashes came as the authorities admitted irregularities in the disputed presidential poll had occurred on a much wider scale than previously disclosed.

The Revolutionary Guard – Iran's most powerful military force – ordered demonstrators to "end the sabotage and rioting activities" and said their resistance was a "conspiracy" against Iran.

A statement posted on the guards' website warned protesters to "be prepared for a resolution and revolutionary confrontation with the guards, Basij and other security forces and disciplinary forces".

Earlier, Iran's powerful guardian council, which last week agreed to investigate some voting complaints, admitted irregularities had been found in 50 constituencies, far more extensive than had been admitted.

A council spokesman, Abbas Ali Kadkhodaei, was quoted on the state TV website as saying the investigation showed more votes had been cast than there were registered voters in these constituencies.

He denied this had any effect on the result, a landslide victory for the incumbent president, Mahmoud Ahmadinejad, over Mir Hossein Mousavi.

"Statistics provided by the candidates, who claim more than 100% of those eligible have cast their ballot in 80-170 cities, are not accurate – the incident has happened in only 50 cities," Kadkhodaei said.

The admission that there were problems in "only 50 cities" was followed by today's strong warning from the Revolutionary Guard, which in effect dared protesters to show their faces on the streets after Mousavi continued to defy an injunction against street protests by the supreme leader, Ayatollah Ali Khamenei.

"The country belongs to you … protesting [against] lies and fraud is your right," Mousavi, who claims Ahmadinejad won re-election through fraud, said in a statement on his website.

In a significant toughening of rhetoric, a senior Iranian politician raised the spectre of legal action against Mousavi.

"Mousavi's calling for illegal protests and issuing provocative statements have been a source of recent unrest in Iran … such criminal acts should be confronted firmly," Ali Shahrokhi, the head of parliament's judiciary committee, was reported by the semi-official Fars news agency as saying. "The ground is paved to legally chase Mousavi."

The opposition did not hold protests yesterday amid signs it was either pausing for breath or running out of steam. Earlier major demonstrations had thrown the regime off balance.

Young supporters of Mousavi urged people to carry black candles with green ribbons to show solidarity with victims of the recent unrest, their website said.

They also said motorists should turn on their headlights for two hours during the evening rush hour to "show their solidarity with families of martyrs killed in recent events".

The authorities have banned a memorial service for Neda Agha-Soltan, the young woman whose death was captured on YouTube and who has come to symbolise the rebellion.

A scheduled memorial service for Agha-Soltan at the Niloofar mosque, in central Tehran's Abbas Abad district, has been barred in case it turns into a rallying point for the opposition.

Reports suggest her body was only handed over to her family by the authorities on condition that she was buried immediately.

All other mosques in Tehran have also been forbidden from staging services in her memory.

At least 10 people were killed in clashes between demonstrators and police and the Basij militia on Saturday.

Police said 457 people were arrested that day, but did not say how many had been detained throughout a week of turmoil.

Meanwhile, Hasan Qashqavi, an Iranian foreign ministry spokesman, denounced the west for its "racial mentality" and singled out the French president, Nicolas Sarkozy, for criticism.

"How can a western president, like the French president, ask for nullification of Iranian election results?" Qashqavi said. "I regret such comments."

He also accused western countries and media of meddling in Iran's affairs.

Britain, which was condemned by Khamenei as "evil" at the weekend, came under renewed fire from Iranian members of parliament.

"The members of the national security and foreign policy commission had serious criticism towards Britain's stance and asked the foreign ministry to reconsider its ties with that country," a commission spokesman, Kazem Jalali, was quoted as saying by the ISNA news agency.

The Foreign Office said today it was evacuating the families of embassy staff in Iran, and advised against all non-essential travel to the country.

As part of its campaign to muzzle the foreign press, the Iranian government suspended the Dubai-based al-Arabiya network and detained at least two local journalists for US magazines.

Iran yesterday announced the expulsion of the BBC's permanent Tehran correspondent, Jon Leyne, for reporting "false news and ignoring impartiality".

In an apparent show of strength, the country launched air force exercises over the Persian gulf, Iranian state radio reported.

It said the three-day manoeuvre would use Russian-made Sukhoi and Iranian-made Saegheh jet fighters.

The warplanes will fly in Iranian airspace over the gulf and the adjoining gulf of Oman, a transit point for around 40% of the world's crude oil.

• This article was amended on 2 June 2011. The original said Neda Soltani's death was captured on YouTube. That is the name of a different individual. This has been corrected.