"The aim of Putin is not Crimea, but all of Ukraine... His troops massed at the border are ready to attack at any moment," Parubiy told thousands of demonstrators gathered in central Kiev.
"The occupant may cross the border any day. In Putin's morbid imagination, Ukraine should be part of Russia," he said.
In Moscow, Russia's deputy defence minister Anatoly Antonov denied it was massing troops on the Ukrainian border, saying it was observing international agreements.