A glum Iranian supporter of reformist candidate Mir Hossein Mousavi rests his head on his arms Saturday as election results are reported. Iran has declared President Mahmoud Ahmadinejad won re-election. Mousavi has condemned what he called the "manipulation" of the results.

TEHRAN, Iran — Iran declared President Mahmoud Ahmadinejad the winner Saturday of an election that pitted the conservative establishment against candidate with broad backing from the country's youth. Riot police attacked opposition supporters, beating them with clubs and smashing cars.

A statement from Mousavi posted on his Web site condemned what he described as the "manipulation" of election results.

Demonstrators wearing the trademark green color of Mir Hossein Mousavi chanted slogans condemning the results that gave 62.6 percent of the vote to President Mahmoud Ahmadinejad. Protesters set fire to tires outside the Interior Ministry in the most serious unrest in Tehran in a decade.

Witnesses also said a commercial bank elsewhere in the city was set on fire.

Police attacked the demonstrators near the Interior Ministry, where the election results were announced. Police also moved to disperse any large gatherings of people around the city.

An Associated Press photographer saw a plainclothes security official beating a woman with his truncheon.

In another main street of Tehran some 300 young people blocked the avenue by forming a human chain chanted "Ahmadi, shame on you. Leave the government alone."

Iranian authorities said that Mousavi only took 33.75 percent of the vote in a contest that was widely perceived to be much closer than the official results.

Iran's supreme leader, Ayatollah Ali Khamenei, who has final say on all state matters, thanked the people for their record 85 percent participation and warned opposition candidates to "avoid provocative behavior."

"I assume that enemies intend to eliminate the sweetness of the election with their hostile provocation," he said in his televised address.

He called the results a "divine assessment" and called on all the candidates to support the president.

Nationwide, the text messaging system remained down Saturday and several 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.

Ahmadinejad had the apparent backing of the ruling theocracy, which holds near-total power and would have the ability to put the election results on the slow track.

It was unclear how many Iranians were even aware of Mousavi's claims of fraud. Communications disruptions began in the later hours of voting Friday — suggesting an information clampdown. State television and radio only broadcast the Interior Ministry's vote count and not Mousavi's midnight press conference.

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.

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.

The state-run Islamic Republic News Agency reported that Ahmadinejad plans a public address on Sunday in Tehran.

In Tehran, several Ahmadinejad supporters cruised the streets waving Iranian flags out of car windows and shouting "Mousavi is dead!"

Before the count, President Barack 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.