Hard-line incumbent Mahmoud Ahmadinejad was heading for a surprise landslide with nearly 80 per cent of votes counted in Iran 's stormy presidential elections, the Interior Ministry said Saturday. But his pro-reform rival countered that he was the clear victor and accused authorities of fraud. The dispute sharply boosted tensions, raising the possibility of a standoff after an intense monthlong race between the combative president and his main challenger, Mir Hossein Mousavi, who is backed by a growing youth-oriented movement. A large turnout at the polls had boosted victory hopes for Mousavi supporters. At a press conference around midnight, Mousavi declared himself definitely the winner'' based on all indications from all over Iran .'' He accused the Islamic ruling establishment of manipulating the people's vote'' to keep Ahmadinejad in power and suggested the reformist camp would stand up to challenge the results.It is our duty to defend people's votes. There is no turning back,'' Mousavi said, alleging widespread irregularities. Before dawn Saturday, Tehran's streets were deserted, but there were worries of protests by Mousavi supporters if he is declared the loser. Bringing any showdown into the streets would certainly face a swift backlash from security forces. The political chief of the powerful Revolutionary Guard cautioned Wednesday it would crush any revolution'' against the Islamic regime by Mousavi's green movement.'' The Interior Ministry banned all rallies until after the formal announcement of results Saturday. A series of cyber-strikes _ blackouts of text messaging, blocks on pro-Mousavi Web sites and widespread Internet disruptions _ also raised worries that authorities were prepared to exert further pressures on the communications lifelines of the rejuvenated reformist movement. Moments after Mousavi's news conference, Iran 's state news agency IRNA reported Ahmadinejad the winner. For a few hours after, Ahmadinejad supporters weaved through Tehran's streets on motorbikes shouting Allahu Akbar,'' or God is great.'' The messy and tense outcome capped a long day of voting. It was extended for several hours to accommodate a huge turnout that had people waiting for hours at polling stations in blistering heat and nighttime downpours. Mousavi, a 1980s-era prime minister, was counting on an outpouring from what's been called his green tsunami'' _ the signature color of his campaign and the new banner for reformists seeking wider liberties at home and a gentler face for Iran abroad. He raised hopes that a new leadership might embrace President Barack Obama's invitation to open dialogue and take a less confrontation path with the West over Iran 's nuclear program. The heavy turnout was expected to help Mousavi. So the Interior Ministry's partial results overwhelmingly favoring Ahmadinejad came as a surprise. By early Saturday, Ahmadinejad had 64.9 percent and Mousavi had 32.6 percent with 78 percent of all votes counted, said Kamran Daneshjoo, a senior official with the Interior Ministry, which oversees the voting. Based on figures released by the ministry, around 75 percent of the 46.2 million eligible voters went to the polls. Mousavi appealed to Iran 's supreme leader, Ayatollah Ali Khamenei, to intervene and stop what he said were violations of the law. Khamenei holds ultimate political authority in Iran . I hope the leader's foresight will bring this to a good end,'' Mousavi said. Mousavi said some polling stations were closed early with people still waiting to vote, that voters were prevented from casting ballots and that his observers were expelled from some counting sites. Authorities should not assume that by manipulating people's vote and staying in power for a day, for a year or two, (they) can win people's satisfaction,'' he said. During the voting, some communications across Iran were disrupted. Internet connections slowed dramatically in some spots, hindering the operations of news organizations including The Associated Press. It was not immediately clear what had caused the disruptions. About a dozen Ahmadinejad supporters pelted a Mousavi office in Tehran with tear gas canisters, but no one was injured, said Saeed Shariati, head of Mousavi's Web campaign. The attack could not be independently confirmed. Iran does not allow international election monitors. During the 2005 election, when Ahmadinejad won the presidency, there were some allegations of vote rigging from losers, but the claims were never investigated. Iran 's ruling clerics 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. Still, within those bounds, Iran 's elections are among the few in the Middle East that can see surprises _ and this year's campaign riveted the world's attention with its wide-open passions and Western-style tactics, including a savvy Web campaign and all-night street parties by Mousavi's young backers. The outcome will not sharply alter Iran 's main policies or sway major decisions, such as possible talks with Washington or nuclear policies. Those crucial issues rest with the ruling clerics headed by the unelected Khamenei. But the election focused on what the office can influence: boosting Iran 's sinking economy, pressing for greater media and political freedoms, and being Iran 's main envoy to the world. Only weeks ago, Ahmadinejad (Ah-mad-in-A-jad) seemed ready to coast to re-election with the reformist ranks in disarray. But Mousavi's bid began to gain traction with young voters with his Web outreach and hip green'' rallies. Suddenly, the 67-year-old Mousavi (Mou-sa-VI) became the surprise hero of a powerful youth-driven movement and heading into the vote, it looked like the momentum was with him. In Washington, Obama said the robust debate'' during the campaign suggests a possibility of change in Iran , which is under intense international pressure over its nuclear program. Ultimately the election is for the Iranians to decide,'' said Obama. But ... you're seeing people looking at new possibilities. And whoever ends up winning the election in Iran , the fact that there's been a robust debate hopefully will help advance our ability to engage them in new ways.'' The intensity was reflected by a rush to the polls on Friday. Some waited for hours in temperatures that hit 113 degrees (45 C) in Iran 's central desert. In Tehran, a bride in her wedding gown cast her ballot. Families making traditional Friday visits to relatives' graves filed into polling stations in the capital's sprawling cemetery. After casting her ballot at a mosque in central Tehran, 23-year-old Mahnaz Mottagh said: I hope to defeat Ahmadinejad today.'' Outside the same polling station, 29-year-old Abbas Rezai said he, his wife and his sister-in-law all voted for Ahmadinejad . We will have him as a president for another term, for sure,'' he said. Iranians around the world _ including southern California and elsewhere in the United States _ also took part in the vote. In Dubai, home to an estimated 200,000 Iranians, the streets around the polling station at the Iranian consulate were jammed with voters overwhelmingly favoring Mousavi. He is our Obama,'' said Maliki Zadehamid, a 39-year-old exporter in Dubai. In Tehran's affluent northern districts _ which strongly back Mousavi _ voters waited for up to an hour to cast ballots. Mahdi Hosseini, a university student, sharply criticized Ahmadinejad for degrading Iran 's image in the eyes of the world.'' Ahmadinejad has brought international condemnation with his repeated questioning of the Holocaust. Mousavi also hammered him over mismanaging the economy, burdened by double-digit inflation and chronic unemployment despite vast oil and gas riches. Mousavi's stunning rise was also helped by his popular and charismatic wife, former university dean Zahra Rahnavard, and their joint calls for more rights and political clout for women. The race will go to a runoff on June 19 if no candidate receives a simple majority of more than 50 percent of the votes cast. Two other candidates _ conservative former Revolutionary Guard commander Mohsen Rezaei and moderate former parliament speaker Mahdi Karroubi _ only got a fraction of the votes, according to the Interior Ministry's results.