Iranian President Mahmoud Ahmadinejad's main challenger on Sunday called on authorities to cancel Friday's presidential election, saying it is the only way to restore public trust. Mir Hossein Mousavi, who has accused authorities of election fraud, urged his supporters to continue their "civil and lawful" opposition to results showing Ahmadinejad won by a landslide. He advised police to stop violence against protesters. Mousavi's statement was posted Sunday on one of his campaign Web sites. It is his first since two days of violent clashes between Ahmadinejad opponents and police erupted on the streets of Tehran. Mousavi has claimed he was the true winner of the election. In a second day of violence Sunday, protesters set fires and smashed store windows as groups challenging Ahmadinejad's re-election tried to keep pressure on authorities that have responded with anti-riot squads and blackouts of Web networks used to rally the pro-reform campaign. Ahmadinejad dismissed the unrest - the worst in the decade in Teheran - as "not important" and insisted the results showing his landslide victory on Friday were fair and legitimate. A huge rally in his support was organized even as clashes flared around the capital. The violence spilling from the disputed results has pushed Iran's Islamic establishment to respond with sweeping measures that include deploying anti-riot squads around the capital and cutting mobile phone messaging and Internet sites used by the campaign of Ahmadinejad's election rival Mousavi. There's little chance that the youth-driven movement could immediately threaten the pillars of power in Iran - the ruling clerics and the vast network of military and intelligence forces at their command - but it raises the possibility that a sustained and growing backlash could complicate Iran's policies at a pivotal time. President Barack Obama has offered to open dialogue after a nearly 30-year diplomatic freeze. Iran also is under growing pressure to make concessions on its nuclear program or face possible more international sanctions. In Paris, French Foreign Minister Bernard Kouchner said he was "very worried" about the crackdown on protesters. "France regrets that instead of openness, there has been quite a brutal response. ... This will leave its mark, and the opposition will organize itself." In a second day of clashes, scores of young people shouted "Death to the dictator!" and broke the windows of city buses on several streets in central Teheran. They have burned banks, trash bins and piles of tires used as flaming barricades to block police. Riot police beat some of the protesters with batons while dozens of others holding shields and motorcycles stood guard nearby. Shops, government offices and businesses closed early as tension mounted. Along Teheran's Vali Asr St. - where pro-Mousavi activists held a huge pre-election rally last week - tens of thousands of people marched in support of Ahmadinejad, waving Iranian flags and shouting his name. In a news conference, Ahmadinejad called the level of violence "not important from my point of view" and likened it to the intensity after a soccer match. "Some believed they would win, and then they got angry," he said. "It has no legal credibility. It is like the passions after a football match. ... The margin between my votes and the others is too much and no one can question it." About a mile away from Ahmadinejad's news conference, young Iranians set trash bins, banks and tires on fire as riot police beat them back with batons. "In Iran, the election was a real and free one," said Ahmadinejad. "The election will improve the nation's power and its future," he told a packed room of Iranian and foreign media. Ahmadinejad also accused foreign media of launching a "psychological war" against the country. Iranian authorities have asked some foreign journalists - in Iran to cover the elections - to prepare to leave. Nabil Khatib, executive news editor for Dubai-based news network Al Arabiya, said the station's correspondent in Teheran was given a verbal order Sunday from Iranian authorities that the office will be closed for one week. No reason was given for the order, but the station was warned several times Saturday that they need to be careful in reporting "chaos" accurately. Iran restored cell phone service that had been down in the capital since Saturday. But Iranians could not send text messages from their phones, and the government increased its Internet filtering in an apparent attempt to undercut liberal voices. Social networking sites including Facebook and Twitter were also not working. The restrictions were likely intended to prevent Mousavi's supporters from organizing large-scale protests. But smaller groups assembled around the city. About 300 Mousavi supporters gathered outside Sharif University, chanting "Where are our votes?" About a dozen riot police used batons to disperse about 50 Mousavi supporters standing outside his campaign quarters. On Saturday, Mousavi, a 67-year-old former prime minister, released a Web message saying he would not "surrender to this manipulation." Authorities responded with targeted detentions, apparently designed to rattle the leadership of Mousavi's "green" movement - the trademark color of his campaign. The detentions include the brother of former reformist President Mohammad Khatami and two top organizers of Iran's largest reformist party, the Islamic Iran Participation Front: the party's secretary-general and the head of Mousavi's youth cyber campaign. Mohammad Reza Khatami and the two party activists were released Sunday. Several others linked to Mousavi's campaign remained in custody, but the full extent of the arrests were not known. Teheran deputy prosecutor, Mahmoud Slarkia, told the semi-official ISNA news agency that fewer than 10 people were arrested on the charge of "disturbing public opinion" through their "false reports" on Web sites after the election. He did not mention any names. Iran's deputy police chief, Ahmad Reza Radan, told the official Islamic Republic News Agency that about 170 people have been arrested for their involvement in Saturday's protests. He said 10 of those arrested were "main planners" and 50 were "rioters." The others were arrested for being at the site of the clashes, he said. Some of the detained were active in Mousavi's campaign headquarters or had relations with foreign media, he said. "Police will not allow protesters to disturb the peace and calmness of the people under the influence of foreign media," Radan said on state television, which showed footage of the protests for the first time Sunday. Mousavi's newspaper, Kalemeh Sabz, or the Green Word, did not appear on newsstands Sunday. An editor, who spoke on condition of anonymity because of the sensitivity of the situation, said the paper never left the printing house because authorities were upset with Mousavi's statements. The paper's Web site reported that more than 10 million votes in Friday's election were missing national identification numbers similar to US Social Security numbers, which make the votes "untraceable." It did not say how it knew that information. "Don't worry about freedom in Iran," Ahmadinejad said at the news conference after a question about the disputed election. "Newspapers come and go and reappear. Don't worry about it." Iran's supreme leader, Ayatollah Ali Khamenei, closed the door for possible compromise. He could have used his near-limitless powers to intervene in the election dispute. But, in a message on state TV on Saturday, he urged the nation to unite behind Ahmadinejad, calling the result a "divine assessment." The US has refused to accept Ahmadinejad's claim of a landslide re-election victory said it was looking into allegations of election fraud. There are no independent election monitors in Iran. US Secretary of State Hillary Rodham Clinton said Saturday she hoped the outcome reflects the "genuine will and desire" of Iranian voters. The European Union also said it was "concerned about alleged irregularities" during Friday's vote. In Beirut, Hizbullah - which is aided by Iran - congratulated Ahmadinejad and said the vote was conducted in an atmosphere of "freedom."