Iran's government has declared President Mahmud Ahmadinejad the decisive winner in the June 12 election.
The Interior Ministry announced that Ahmadinejad won 62.63 percent of the vote, while the top challenger, Mir Hossein Musavi, won 33.75 percent.
Officials say turnout topped 85 percent, the highest ever for a national election in the Islamic Republic.
Iran's Supreme Leader, Ayatollah Khamenei, who has the final say in all state matters, praised Ahmadinejad's re-election. In a statement, he said Ahmadinajad's landslide victory was a "real feast which can guarantee the country's progress, national security and lasting joy."
Khamenei went on to say that the election was a "massive success" and he urged everyone to be grateful for this "divine blessing."
But Musavi, who last night claimed victory for his camp, says there were clear "irregularities." In an open statement addressed to the Iranian people he described the announced results as "shocking" and called on officials to acknowledge the people's choice.
Musavi said he will not surrender to what he termed a "dangerous charade." He urged his supporters to resist what he called a "governance of lies and dictatorship."
Musavi was due to have a news conference on the afternoon of June 13 in Tehran, but the event was cancelled. Musavi said on his website that the news conference would be rescheduled for another day.
The two other presidential candidates, conservative Mohsen Rezai and reformist cleric Mehdi Karrubi, received about 3 percent of the vote, according to the official tally.
Karrubi has joined Musavi in saying he does not accept the official results.
Protests In Tehran
After a mostly quiet morning in Tehran, with a heavy police presence around government buildings, some Musavi supporters began taking to the streets of the Iranian capital. By early afternoon, several thousand protestors had assembled chanting: "The government lied to the people!" and "Musavi, take back our vote!"
Some demonstrators set fire to car tires outside the Interior Ministry. Others blocked a main street, forming a human chain.
Still, it appears most opposition supporters are holding back. Many are instead flooding social networking site Facebook with messages expressing their shock and dismay at what they view as the authorities' hijacking of the election.
One witness to today's demonstrations, a young man from Tehran, told RFE/RL by telephone that "the atmosphere in Tehran is tense and convulsive [but at the same time] there is a very meaningful silence and I can say that Tehran does not believe the results that have been announced by the Interior Ministry."
Mehdi Arabshahi, one of the leaders of Iran's largest reformist student group, told Radio Farda that he has serious doubts about the official results.
"So far they have announced that Ahmadinejad has about 20 million votes, but based on what we saw [in Tehran] and reports we had from other cities and even villages, it was the opposite of what was announced -- meaning that all were pretty sure that another candidate would win in the first round. And if not, they were expecting a runoff," Arabshahi said.
Unconfirmed reports say that Musavi, Karrubi, and Rezai, along with former president Mohammad Khatami, have either already met with Khamenei or have requested a meeting. Khamenei implicitly expressed his support for Ahmadinejad ahead of balloting.
Warnings Of Fraud
Ahead of the vote, Karrubi and Musavi had warned of possible election fraud.
Another Tehran-based observer, who did not want to be named, told RFE/RL that many believe there has in fact been massive fraud.
Critics note the record speed with which Ahmadinejad's preliminary victory was announced, along with some highly unusual local results.
For example, according to the official tally, reformist Mehdi Karrubi came in a distant second in his own hometown of Oligudarz, garnering less than half of Ahmadinejad's vote.
Sarejeddin Mirdamadi, a reformist journalist in Tehran, says suspicious movements he observed at the Interior Ministry add to his concerns. The Interior Ministry is in charge of counting the votes.
"I wanted to enter the Interior Ministry but they prevented me, even though I had an entry pass," Mirdamadi said. "There I saw that they also prevented a number of the staff who wanted to go to work from entering the building. Later one of them called me and said that a group of personnel from different divisions of the Ministry were not allowed to enter the building."
The official results have also staggered independent Iran watchers.
Karim Sadjadpour, a senior Iran analyst at the Carnegie Endowment for International Peace, told Reuters that no one expected this level of "fraudulence."
Abolhassan Bani Sadr, the first president of Iran following the 1979 revolution, spoke to Radio Farda from his home in Paris. He went even further, recalling the words of Adolf Hitler's chief propagandist, Joseph Goebbels, who famously said: "If you tell a lie big enough and keep repeating it, people will eventually believe it."
"In my view this is a massive fraud; there is nothing vague about it," he said.
In the run-up to the June 12, vote many young people had been holding street parties to express support for their candidates.
Many were hoping for change, especially the legions of young people and women sporting Musavi's green campaign color in a public show of support. Some of them are now expressing huge disappointment and frustration on social networking sites like Facebook and Twitter.
Some have posted a photo of a burning Iranian map on their Facebook page, while others are calling today "Black Saturday," and "bidding farewell to hope."
Radio Fardas Mahin Gorji and Alireza Taheri contributed to this report.