Russian troops launched a wide-ranging attack on Ukraine on Thursday, as President Vladimir Putin cast aside international condemnation and sanctions and warned other countries that any attempt to interfere would lead to “consequences you have never seen.”

Big explosions were heard before dawn in Kyiv, Kharkiv and Odesa as world leaders decried the start of an invasion that could cause massive casualties, topple Ukraine’s democratically elected government and threaten the post-Cold War balance on the continent.

Ukrainians started fleeing some cities, and the Russian military claimed to have incapacitated all of Ukraine’s air defenses and air bases within hours.

President Volodymyr Zelenskyy declared martial law, saying Russia has targeted Ukraine’s military infrastructure. Ukrainians who had long braced for the prospect of an assault, while never knowing precisely when it would come, were urged to stay home and not to panic even as the country's border guard agency reported an artillery barrage by Russian troops from neighboring Belarus.