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Hardline incumbent Mahmoud Ahmadinejad was on Saturday declared winner by a landslide in Iran's hotly-disputed presidential vote, triggering riots by opposition supporters and furious complaints of cheating from his defeated rivals.
Ahmadinejad went on television to declare the election a "great victory," even as baton-wielding police were clashing with protestors in the streets of the capital in unrest not seen since student riots a decade ago.
Thousands of supporters of Mousavi swept through Tehran shouting "Down with the Dictator" after final results showed Ahmadinejad winning almost 63 percent of the vote.
Moderate ex-premier Mousavi cried foul over election irregularities and warned the outcome of the vote could lead to "tyranny," as some of his supporters were beaten by riot police.
Ahmadinejad however rejected the allegations.
"The election was completely free... and it is a great victory," he said in his television address, adding that through the vote Iranians had "inspired other nations and disappointed their ill-wishers."
Thanking all his supporters for creating "this huge epic," Ahmadinejad said he intends in his new term of office to walk the same path but with a "higher and wider perspective."
He also called on his supporters to gather on Sunday at 5:00 pm (1230 GMT) in the capital's Vali Asr Square, where many of Saturday's clashes occurred.
Even as he was speaking, Iran's main cellular phone network was cut while social networking site Facebook was also blocked.
The interior minister said Mousavi had won less than 34 percent of the vote, giving Ahmadinejad another four-year term in a result that dashed Western hopes of change and set the scene for a possible domestic power struggle.
Iran's all-powerful supreme leader Ayatollah Ali Khamenei hailed Ahmadinejad's victory and urged the country to unite behind him after the most heated election campaign since the Islamic revolution in 1979.
The vote outcome appears to have galvanised a grass-roots movement for change after 30 years of restrictive clerical rule in a country where 60 percent of the population was born after the revolution.
The international community had also been keenly watching the election for any signs of a shift in policy after four years of hardline rhetoric from the 52-year-old Ahmadinejad and a standoff over Iran's nuclear drive.
Mousavi protested at what he described as "numerous and blatant irregularities" in the vote which officials said attracted a record turnout of around 85 percent of the 46 million electorate.
"No one can imagine such rigging, with the world watching, from a government who holds commitment to shariah-based justice as one of its basic pillars," said Mousavi said in a letter posted on his campaign website.
"What we have seen from dishonest (election) officials will result in shaking the pillars of the Islamic republic system, and a dominance of lying and tyranny," he said in a separate statement.
In the heart of Tehran, Mousavi's supporters voiced their disbelief and frustration at the results, with some throwing stones at police who struck back with batons.
Angry crowds first emerged near Mousavi's campaign office in central Tehran, where protestors, including women, were hit with sticks as riot police on motorbikes moved in to break up the gathering, an AFP correspondent said.
Late Saturday police further beefed up their presence in main streets and squares, especially in the area housing Mousavi's office, while dozens of men were seen handcuffed and detained in an interior ministry compound.
Members of Iran's volunteer Basij militia were also being deployed in some parts of the city while several smouldering garbage cans were seen lying on the sidewalks after being set ablaze by rioters.
Reformist candidate Mehdi Karroubi, who came a distant fourth with less than one percent of the vote after ex-Revolutionary Guards chief Mohsen Rezai in third, also declared the result "illegitimate and unacceptable."
"They have ruined the country and they want to ruin it more over the next four years," shouted an irate mob outside Mousavi's office.
The White House said Saturday it was monitoring the reports of irregularities, while British Foreign Secretary David Miliband said London "will continue to follow developments" in Iran.
In Moscow, the chairman of the Duma (parliament) Committee on International Affairs Konstantin Kosachev hoped for more "understanding and wisdom" from Ahmadinejad in the new term.
"The results of the election show, now more than ever, how much stronger the Iranian threat has become," said arch-foe Israel's deputy foreign minister Danny Ayalon.
Mousavi had been hoping for a political comeback on a groundswell of support among the nation's youth, with pledges to ease restrictions particularly on women, and fix Iran's ailing economy.
The election highlighted deep divisions in Iran after four years under Ahmadinejad, who had massive support in the rural heartland, while in the big cities young men and women threw their weight behind Mousavi.
