Following a night of violent protest in which thousands took to Tehran's streets, Iran's President Mahmoud Ahmedinejad insisted Sunday his re-election was not a fraud, according to dpa. Ahmadinejad also insisted there would be no change in Iran's nuclear policies, and he challenged US President Barack Obama to a debate on the nuclear dispute. The 52-year-old was addressing a press conference for the first time since Friday's vote after which his main challenger, Mir-Hossein Moussavi, accused the president and interior ministry of having cheated in the counting process. "The election was free and there is no document proving these charges," Ahmadinejad said. "It is really ridiculous that the loser of the election claims a majority of the votes belong to him." Moussavi said in a statement on his website that he had forwarded an official request to Iran's Guardian Council asking it to nullify the results. Besides Moussavi, several opposition parties and groups have as well doubted the election results and even an influential clergy group has called on new elections. Also on Sunday, US Vice President Joe Biden weighed in, saying that the United States finds the recent Iranian re-election of President Mahmoud Ahmadinejad "questionable," but would accept the results "for the time being." Police meanwhile confirmed dozens of demonstrators were arrested in overnight clashes in Tehran. The official news agency IRNA quoted deputy police chief Ahmad Radan as saying ten "organizers" and 50 demonstrators were held and more arrests would follow. A number of demonstrators were reported injured in the clashes, but owing to a news embargo in the local media no clear details were immediately available. Clashes began again Sunday as tens of thousands of Ahmadinejad supported celebrated in downtown Tehran, while Moussavi supporters gathered in the northern part of the capital in a counter-march. Addressing crowds chanting "Eyval, eyval, Ahmadi(nejad), eyval" (well done, well done, Ahmadinejad), the president again rejected fraud charges by the opposition, and claimed Friday's election had been even more democratic than in the West. "There is not one single document, not even by the other candidates, protesting against the election process," Ahmadinejad said. He accused the opposition and Western press of wrongly reporting about election fraud, saying the West only wanted democracy when the result suited them, too. "Their (Western press) standpoint is if the candidate they propagated does not win, then there has definitely been cheating," he said. "In the West they even seek support from homosexuals to get a few more votes. In Iran however our democracy is based on ethics." Ahmadinejad was defiant at his news conference that there would be no change in Iran's nuclear policies. "Negotiations on the nuclear issue (suspension of uranium enrichment) are history," he said. "As I said last month, I am ready to have a debate with the American president," he said. He had expressed readiness last month to debate with Obama at UN headquarters in New York. Iran insists that its nuclear projects are solely for civil and peaceful purposes, but the West fears that Tehran would avail itself of its nuclear know-how to pursue a secret military programme. Referring to Saturday night's street unrest, Ahmadinejad said there was no crisis, and compared the protestors to football fans whose team had lost and who could not tolerate the defeat. "That is natural, these are short-term emotional reflections," he said, terming the demonstrators a minority compared to the majority who had voted in his favour. The labour news agency ILNA earlier quoted deputy police chief Radan as categorically denying reports that Moussavi and his wife, Zahra Rahnavard, had been put under house arrest in order to avoid further unrest. However, eyewitnesses said that despite tight police control, sporadic protest gatherings had again started Sunday in several parts of Tehran with Moussavi supporters shouting "Death to the dictator" and "We want our votes back." Thousands of opponents of Ahmadinejad took to the streets on Saturday setting off a cycle of protests and violence after the Interior Ministry announced Ahmadinejad had won Friday's election with more than 62 per cent of votes. Opposition circles said several members of the reformist party Islamic Iran Participation Front which supported Moussavi in the election were arrested. The judiciary denied there had been arrests, but said that some were "summoned" to the judiciary. Some disruption of internet links remained Sunday after SMS and mobile telephone networks, heavily used by opposition supporters in the runup to the election, were switched off for a while Saturday. Tehran witnessed protests before during the student-led riots of 1999, but observers said Saturday's demonstrations were bolder. Moussavi said on his website he would not "surrender to the manipulation." With communications disrupted, few details emerged of those injured in the clashes - but television reaching the outside world showed police making heavy use of sticks to beat demonstrators. While police banned demonstrations and cancelled a press conference for foreign correspondents, the protesters - mainly Moussavi supporters - refused to back down. Supreme Leader Ayatollah Ali Khamenei had earlier confirmed the election result and congratulated Ahmadinejad. "The elected president is the president of all Iranians and also his opponents should now support and help him," he said in a message on state television. "With their presence, the people turned the election into an epic day," he said, referring to the 85-per-cent record turnout. Meanwhile, an influential Iranian clergy group said the counting process was fraudulent and called for the nullification of the election. The Islamic Combatant Clergy Association (ICCA), which is close to former president Khatami and supported Moussavi in Friday's vote, said on its website that the election should be held again in a calmer, more logical and more just atmosphere. The Interior Ministry has rejected all fraud charges. Interior Minister Sadeq Mahsoul said Ahmadinejad got 24.5 million votes and secured himself an absolute majority of 62 per cent. Moussavi received 33.7 per cent - 13.2 million - of the votes.