A peaceful handover of power has reportedly been agreed in the Ukraine after protesters clashed with anti-riot police outside the president's headquarters. Tensions in the capital Kiev reached breaking point as tens of thousands of demonstrators surrounded the HQ. They had been called on to march by opposition leader Viktor Yushchenko. He and his supporters believed the presidential election, which took place at the weekend, was rigged. According to the poll results, Kremlin-backed Prime Minister Viktor Yanukovych won the presidency. But after calls from outgoing president Leonid Kuchma for talks between the two sides, Mr Yanukovych stepped aside. Mr Yushchenko will now become president, it has been reported. The turmoil in the Ukraine followed a day of claims and counter-claims about the disputed presidential election, which has been condemned internationally as not being "free or fair". But Russian President Vladimir Putin said any doubts expressed by the international community were "inadmissible". He said supporters of both candidates in the disputed election should act within the law. "Everything must remain legal," he said. "Ukraine is a big democratic state with a developed democratic system. There is no need to teach it democracy." Sky's Moscow correspondent Laurence Lee described the situation in Kiev as "very tense". The Organisation for Security and Cooperation in Europe had expressed concern at the poll. Senator Richard Lugar, chairman of the US Senate's Foreign Relations Committee, said there had been a "concerted and forceful programme of election day fraud and abuse".