Iranian women stand in line Friday to cast their votes in the election at the Masoumeh shrine in Qum, about 120 kilometres south of Tehran. (Kamran Jebreili/Associated Press)Hardline incumbent Mahmoud Ahmadinejad has pulled far ahead in Friday's hotly contested Iranian presidential election, according to the country's Interior Ministry, though his biggest rival is also claiming victory.

By early Saturday, Ahmadinejad had 65.2 per cent and reformist candidate Mir Hossein Mousavi had 31.8 per cent with 77 per cent of all votes counted, said Kamran Daneshjoo, a senior official with the ministry, which oversees the voting.

Iranian President Mahmoud Ahmadinejad waves to his supporters after he cast his ballot Friday in Tehran. (Ahmed Jadallah/Reuters) Earlier, Mousavi declared himself "definitely the winner," based on "all indications from all over Iran," but offered no support of his victory claim at a news conference in Tehran. He alleged widespread irregularities at the polls.

Moments after Mousavi called the news conference, Iran's state news agency reported that Ahmadinejad was the victor. Both claims came shortly after polls closed but before any vote tallies were announced.

Voting hours were extended to accommodate a huge turnout at polls across the country.

One of Mousavi's campaign organizers accused Ahmadinejad supporters of attacking one of his opponent's campaign offices in Tehran with tear gas.

Saeed Shariati, head of Mousavi's internet youth campaign, told The Associated Press that police quickly dispersed the group. There was no independent confirmation of the attack.

Crowds formed quickly at many voting sites in areas considered strongholds for both men.

Based on figures released by the ministry, around 75 per cent of the 46.2 million eligible voters went to the polls

An estimated 45,000 polling stations opened countrywide Friday morning. State television encouraged people to vote and broadcast video footage of past elections, nationalistic songs and pictures of Iranians fighting against Iraq in the 1980-88 war.

"We've been at this poll since early this morning and the line has only grown longer — in fact it's now going all the way around the corner," said the CBC's Nahlah Ayed, reporting from a polling station in Tehran.

Accusations of improprieties

Iran's cellphone text messaging system was down Friday. Many Iranians, especially the young voters who tend to favour Mousavi, have frequently used text messages to spread election information to friends and family.

Presidential candidate Mir Hossein Mousavi raises his inked finger and identity card behind his wife, Zahra Rahnavard, after casting his vote in Tehran on Friday. (Raheb Homavandi/Reuters)Mousavi, Iran's former prime minister, accused Iran's government-owned telecommunications provider of shutting down the system and alleged that some of his representatives were barred from entering polling stations to monitor the vote, which could be a close one by some accounts.

"Unfortunately, some of my representatives were blocked from entering polling stations and SMS [text messaging] is also down, which is against the law," Mousavi said after voting, according to his campaign website. "We should not be fearful about the free flow of information, and I urge officials to observe the law."

The text messaging system has been down since late Wednesday, said Telecommunication Ministry spokesman Davood Zareian.

A top Mousavi aide, Ali Reza Beheshti, said some polling stations in northwestern and southern provinces ran out of ballots, claiming it was a "deliberate attempt by the government to keep people from voting."

Iran's elections are considered generally fair, but the country does not allow international monitors. The ruling clerics, however, put their stamp on the elections from the very beginning by deciding who can run.

More than 470 people sought to join the presidential race, but only Ahmadinejad and three rivals were cleared.

The rowdy campaign reached a crescendo in the past few days with duelling rallies by supporters of Ahmadinejad and Mousavi that drew tens of thousands into the streets of Tehran.

Fervent, youthful supporters of Mousavi accused the president of undermining Iran's international standing with his confrontational style and of devastating the economy.

Supreme Leader Ayatollah Ali Khamenei cast his ballot minutes after the polls opened, as is customary at presidential elections. He urged voters to vote early.

"As far as I see and hear, passion and motivation is very high among people," said Khamenei. "We expect people at packed polling stations to be able to choose the best choice for heading the executive branch for four years."

Possible runoff

There are four candidates in the running — former Revolutionary Guard commander Mohsen Razai and former parliamentary Speaker Mehdi Karroubi trail the leading two candidates. The winning candidate has to secure more than 50 per cent of the vote, which many analysts say is unlikely in the first round of voting.

Mir Hossein Mousavi's supporters rally at the Heidarnia stadium in Tehran on Tuesday. (Ben Curtis/Associated Press)

If none of the candidates can secure this margin, a runoff election between the top two vote-getters will be held June 19.

Consequently, the support of the two trailing candidates could be crucial to the front-runners. Razai is expected to draw conservative voters, and Karroubi has a largely moderate base.

Ahmadinejad, meanwhile, has broad support among rural and working poor voters, while Mousavi has much of the middle-class and urban vote. The newly elected president will serve a four-year term.

Mousavi looked to be mounting a strong challenge to Ahmadinejad, whose once high approval ratings flagged with a sinking economy. Mousavi has the backing of former president Mohamed Khatami, who was elected in 1997 and 2001.

Mousavi has criticized Ahmadinejad's foreign policies as provocative, and says he will seek to better Iran's relationship with the international community, particularly with its neighbours in the Middle East and the United States. U.S. President Barack Obama has spoken of improving ties with the Islamic nation.

All the candidates are in favour of continuing Iran's national nuclear program, which has given many citizens a sense of national pride. None of the candidates recognize Israel as a nation.

Regardless of who wins, the outcome is expected to have at best a modest impact on issues like the nuclear development program and foreign policy at large.

As supreme leader, Khamenei has the final word on all state matters, as mandated since 1979's Islamic revolution. But analysts say he may show flexibility if there is broad public support for a moderate agenda.

Still, the president has influence over some domestic affairs, such as the economy, and serves as Iran's highest-ranking envoy on the international stage.

Sudden surge

Political scientist Sadegh Zibakalam told the CBC's Margaret Evans he finds it hard to explain the surge in support for Mousavi, Iran's prime minister from 1980 to 1988.

Mousavi has maintained a low profile in recent years.

"Who knows him, to begin with? The man has been prime minister [but] during the past 20 years no one's heard of him."

But he believes Mousavi's challenge may be too strong for Ahmadinejad to withstand.

"It appears that an earthquake, a flood, an avalanche is coming that is much more powerful than the one that brought Khatami to power.

"We are surprised, we are baffled, but it is coming."

With files from The Associated Press