Supporters of Opposition candidate Viktor Yushchenko at a protest rally in Kiev. (AFP)

Kiev, Nov. 22 (Reuters): More than 100,000 Ukrainians poured into the streets of their capital today to protest the result of a presidential election they say was stolen by backers of Prime Minister Viktor Yanukovich.

The ex-Soviet state?s nationalist western regions also rebelled, saying they would recognise only liberal rival Viktor Yushchenko as the rightful president.

Anger erupted among supporters of Yushchenko, a West-leaning liberal, after figures by electoral authorities showed Yanukovich, the establishment candidate backed by Moscow, winning with almost all the votes counted. Ukraine?s security bodies warned they would put down any lawlessness ?quickly and firmly?.

?We appeal to the organisers of mass protests to assume responsibility for their possible consequences,? said a statement issued by the prosecutor general, the interior ministry and the security services. US and western monitors also denounced the vote as undemocratic. But Russian President Vladimir Putin congratulated Yanukovich today, Interfax news agency reported, exposing the differences between Moscow and the West over Ukraine.

The rebellion from the western regions was an ominous sign that sharpened the traditional divide between its Russian-speaking east, behind Yanukovich, and Ukrainian nationalist regions in the west, backing Yushchenko.

A top White House representative said authorities had indulged in a ?concerted and forceful programme? of fraud. Ukrainian envoys in EU countries were being summoned to hear expressions of concern.

Earlier, Yushchenko, who narrowly won a first round of voting but failed to win a majority, said moves were afoot by the authorities to cheat him of victory. The election was seen as a decisive moment in Ukraine's post-Soviet history, offering the choice of tilting towards the West or leaning to Russia.