At least 100,000 Ukrainians, staging the biggest demonstration of post-Soviet times, poured through Kiev on Saturday to demand a clean presidential election in a show of strength for opposition hopeful Viktor Yushchenko. Tens of thousands of supporters carrying the orange banners of Yushchenko's campaign surged down Kiev's elegant central streets bringing traffic to a halt. "A decisive day for the nation is approaching. We are ready. We demand fair elections," Yushchenko told a roaring crowd outside the building where officials will count the votes in the election's first round on Oct. 31. "We need only one thing -- new leadership. Leaders who will open a door to your future." Yushchenko, a liberal pro-Western former prime minister and central banker, is expected to finish atop of the poll along with Prime Minister Viktor Yanukovich, who has the backing of outgoing President Leonid Kuchma. A run-off in November is all but inevitable, given a first-round field of 24 candidates . The United States and European Union have called on Ukraine to ensure a fair poll, seen as an assessment of Kuchma's decade in power, plagued by scandals and accusations of corruption. Yushchenko has based his campaign on advancing political and economic reform to move closer to Ukraine's western neighbours, new members of the European Union. Yanukovich, a former governor in industrial eastern Ukraine. has pledged to consolidate ties with Russia, tighten state control of the economy and make Russian an official language -- all anathema to the country's large nationalist constituency.