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Iranian President Mahmoud Ahmadinejad, left, attends a meeting at the presidency in Tehran, on Tuesday. He will be sworn in sometime between July 26 and Aug. 19. ((ISNA, Alireza Sotakbar/Associated Press))

Protesters and riot police clashed in Tehran on Wednesday, according to media reports quoting witnesses, as hundreds of people protesting the country's election results gathered next to Iran's parliament buildings in defiance of government orders to halt such demonstrations.

Black-clad security officers used tear gas and batons on protesters gathered on Baharestan Square as a helicopter hovered over central Tehran. Some reports said the officers fired rounds of ammunition into the air.

During a telephone interview with CNN, one woman who said she witnessed Wednesday's violence described it as a "massacre."

"They were beating people like hell," she said.

Groups of demonstrators fled to another Tehran plaza — Sepah Square — about two kilometres away, according to witnesses.

The reports said thousands of security officers were in the streets around the parliament and appeared to vastly outnumber the protesters.

State-run Press TV said security forces dispersed the crowd of 200, which it referred to as "an illegal rally."

"A heavy presence of the police prevented violence in the area," the report said.

The regime's restrictions on reporters have made it extremely difficult to make independent verications of reports of violence from the country.

The fresh clashes came as the New York-based International Campaign for Human Rights in Iran published a list of 240 people it says it has confirmed as having been detained since the June 12 vote. The rights group said it believes the actual number is much higher.

Won't bow to protesters: supreme leader

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Security personnel are seen sitting in front of the Iranian parliament in Tehran on Wednesday. ((Demotix Images/Reuters))

Earlier Wednesday, Ayatollah Ali Khamenei, Iran's supreme leader, told legislators the government won't give in to pressure over the disputed results, despite announcing that opposition candidates in Iran's presidential election will have five more days to lodge complaints.

"On the current situation, I was insisting and will insist on implementation of the law. That means, we will not go one step beyond the law," Khamenei said on state television.

"For sure, neither the system nor the people will give in to pressures at any price."

But Khamenei, who controls the levers of power in Iran, has accepted a request from the Guardian Council to allow five more days for candidates to enter complaints about the June 12 election that gave President Mahmoud Ahmadinejad a landslide victory.

The 12-member cleric council must ratify the results but has already ruled out the possibility of a new vote, indicating that its investigation into the disputed voting found no major incidents of fraud.

Khamenei has said the presidential election was won fairly and is a definitive victory.

Ahmadinejad's victory by a 2-to-1 margin has spurred allegations of vote-rigging and sent tens of thousands of protesters from both sides into the streets.

The cleric council has acknowledged that voting irregularities — more ballots had been cast than voters registered — had been found in 50 of Iran's 170 districts. But the discrepancies, estimated to be about three million votes, were not widespread enough to impact the election's outcome, the council said.

At least 17 people have been killed in near-daily demonstrations, some with thousands of protesters. Defeated moderate candidate Mahdi Karroubi called on Iranians to hold mourning ceremonies on Thursday for those killed in the protests.

But Khamenei and elite military force, the Revolutionary Guard, have warned against continued protests and cracked down on demonstrators.

Ahmadinejad to be sworn in

State television reported Wednesday that Ahmadinejad will be sworn in sometime between July 26 and Aug. 19.

According to government officials, Ahmadinejad won 62.6 per cent of the votes, while reformist candidate Mir Hossein Mousavi garnered 33.75 per cent in a contest that appeared to divide urban and rural voters.

Complaints already lodged regarding the election results include allegations of shortages of ballots, people on site trying to force citizens to vote for a particular candidate and expelling candidates' representatives from polling stations, the council has said.

Despite the longer window to enter complaints, conservative candidate Mohsen Rezaie, who came third in the poll, said he had withdrawn his complaints for the sake of the country, according to state television.

Mousavi has largely remained out of sight since Thursday and has only posted online statements on his official website.

On the site on Wednesday, Mousavi's wife, Zahra Rahnavard, called for the release of detained Iranians and an end to the crackdown on protesters.

"It is my duty to continue legal protests to preserve Iranian rights," she said.

With files from The Associated Press