Russian President Vladimir Putin's troops are poised to attack Ukraine at any time, Ukraine's National Security and Defence Council chief Andriy Parubiy warned on Sunday.
"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.
"Russia's armed forces are not carrying out any unannounced military activity that could threaten the security of neighbouring states," he added, cited by Interfax news angency.
Read: Russian troops storm Ukraine base in Crimea