Hard-line incumbent Mahmoud Ahmadinejad was heading for a surprise landslide with nearly 80 percent 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. There were worries of protests by Mousavi supporters if he is declared the loser, though there was no sign of gatherings Saturday morning. 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." And it was unclear how many Iranians were even aware of the claims of fraud, amid widespread communications disruptions that began in the later hours of voting Friday - suggesting an information clampdown. State television and radio only broadcast the Interior Ministry's vote count. Nationwide, the text messaging system remained down Saturday and pro-Mousavi Web sites were blocked or difficult to access. Text messaging is frequently used by many Iranians - especially young Mousavi supporters - to spread election news. In Teheran's streets Saturday morning, Iranians heading to work gathered around newspaper stands to read the headlines, which did not specifically declare a victor - or carry word of Mousavi's claims. Mousavi's paper, Kalemeh Sabz, or the Green Word, and other reformist dailies were ordered to change their headlines originally declaring Mousavi the victor, according to editors at the papers, who spoke on condition of anonymity because of the sensitivity of the issue. The papers had blank spots where articles were removed. 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 US President Barack Obama's invitation to open dialogue and take a less confrontational path with the West over Iran's nuclear program. The heavy turnout had been expected to help Mousavi. But moments after Mousavi's news conference, Iran's state news agency IRNA reported Ahmadinejad the winner. After what had been seen as a close contest, the overwhelming margin for Ahmadinejad in the Interior Ministry's partial results was startling. By Saturday morning, Ahmadinejad had 64.7 percent and Mousavi had 32.2 percent with 82 percent of all votes counted, said Kamran Daneshjoo, a senior official with the Interior Ministry, which oversees the voting. Even in Mousavi's hometown province of Tabriz in northwestern Iran, the ministry claimed Ahmadinejad received more than 60 percent of the vote. Based on the ministry's figures, 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 Teheran 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 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 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 Teheran, 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. In Teheran'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.