Police in Iran beat protesters and fired tear gas and water cannons at the thousands demonstrating in open defiance of the government, and reports said more than a dozen people died in the latest day of demonstrations.

Hospital sources told CNN that at least 19 people died on Saturday, in the country's biggest uprising since the 1979 revolution. Other unconfirmed reports suggest the death toll over the past week is up to 150.

U.S President Barack Obama said that Iran needed to halt its "violent and unjust" crackdown on the demonstrators -- his boldest statement yet on the crisis.

"We call on the Iranian government to stop all violent and unjust actions against its own people," Obama said in a written statement. "Suppressing ideas never succeeds in making them go away."

Images and video taken by protesters showed a clear escalation of violence taking place. One disturbing YouTube video uploaded on Saturday showed a young woman dying in Tehran after being shot.

According to the person who uploaded the video, she was shot by a member of the pro-government Basij militia. Unconfirmed reports have blamed the militia for numerous protester deaths.

Canada denies allegations

Numerous posts on Twitter's Iranian election page say that dozens of people have been severely beaten and that foreign embassies in Tehran are taking in the wounded.

There were allegations on the web Saturday that the Canadian embassy was denying entry to injured people seeking sanctuary, but the Department of Foreign Affairs said those allegations were false.

"The Embassy was closed Saturday and there were no Canadians at the Embassy when the protests began. Reports that we were providing shelter to Iranian demonstrators are false," foreign affairs and international trade spokesperson Simone MacAndrew said in a statement to CTV.ca.  

"Canada's Embassy is located in the centre of recent demonstrations. Due to the tense security in Tehran this week, the Embassy has been closing early so that staff can return home safely before the public and democratic demonstrations begin."

Foreign Affairs called the violent crackdown by authorities "wholly unacceptable."

Liberal Leader Michael Ignatieff issued a statement late Saturday night saying, "Canada should join other countries in keeping our embassy open for the humanitarian needs of the people of Iran."

Foreign Affairs says that Canadian embassies do not normally offer asylum to individuals abroad but will provide temporary safe haven if there is an immediate threat or injury.

'Ready for martyrdom'

Opposition leader Mir Hossein Mousavi has demanded Iran's highest electoral authority annul the presidential vote.

According to The Telegram, Mousavi delivered a defiant speech on Saturday, saying he was "ready for martyrdom," and would continue fighting the election result.

In a letter to the Guardian Council, which investigates voting violations, he said thousands of his supporters were turned away from polling stations during the June 12 vote.

"The Iranian nation will not believe this unjust and illegal" act, Mousavi wrote.

He also alleged that some ballot boxes had been sealed before voting began, and that fake ballots were found in mobile polling stations, according to the Associated Press.

The letter was posted on one of his official websites, and came just one day after Iran's supreme leader, Ayatollah Ali Khamenei, defended the election results and warned against illegal protests.

Khamenei declared that President Mahmoud Ahmadinejad's re-election was an "absolute victory" and warned protesters that they would be held responsible for "bloodshed and chaos" if the mass demonstrations continue.

Despite the warning, thousands of protesters continued to rally throughout Tehran on Saturday.

English-language state television in Iran reported that a bomb exploded in Tehran at the shrine of revolutionary founder Ayatollah Ruhollah Khomeini, killing one person and injuring two. But the report could not be independently verified due to a crackdown on both Iranian and foreign journalists.

Elsewhere in Tehran, witnesses told AP that a crowd of about 3,000 protesters near Revolution Square chanted "Death to the dictator" and other slogans before police moved in to fire tear gas and a water canon.

Freelance journalist George McLeod, one of the few foreign journalists left in Iran, said Saturday said he has seen police beat protesters, which left a few of them seriously wounded.

"I've seen bricks thrown, there's smoke coming up, I've heard explosions from probably tear gas canisters," McLeod told CTV News Channel. "This is happening all around the city it seems."

Eyewitnesses said that between 50 and 60 protesters had to be taken to Imam Khomeini hospital in Tehran after being beaten by members of the pro-government Basij militia.

The witnesses reported that the protesters, many wearing the green colour of Mousavi's campaign, retaliated by setting fire to motorcycles that belong to the militia.

The protesters are demanding a new vote after last week's national election gave President Mahmoud Ahmadinejad a landslide victory.

Earlier Saturday, fire trucks parked around Revolution Square, while riot police descended on Tehran University, where protesters and security forces have previously clashed.

Tehran Province Police Chief Ahmad Reza Radan had earlier warned that "police forces will crack down on any gathering or protest rally, which are being planned by some people."

'It's going to get much worse'

Because of the most recent warning against public protests, demonstrators flooded Tehran's streets Saturday disguised as rush hour travellers before they began to rally, according to McLeod.

"I think things in Tehran are going to start to get very, very violent over the next few hours," McLeod said. "They already are violent right now, but I think it's going to get much worse."

The Interior Ministry has suggested that Mousavi, a former prime minister, may "be held responsible for the consequences of any illegal gatherings."

State Security Council secretary Abbas Mohtaj posted a statement on the ministry's website accusing Mousavi of supporting protests that have "lead to the disruption of security and public order."

It is unclear if Mousavi was present at Saturday's protests. His spokesperson said Friday that he was not under house arrest, but is not allowed to talk to journalists or speak at the rallies.

Mousavi had been invited to attend a Saturday meeting of Iran's Guardian Council -- made up of clerics and Islamic law experts -- which oversaw last week's election.

The Council's spokesperson said Mousavi and another reformist candidate, Mahdi Karroubi, did not attend the meeting.

The Council had previously announced it was willing to conduct a recount of some ballots from voting stations where candidates reported irregularities.

With files from The Associated Press