MOSCOW (AP) — Ukrainian President Volodymyr Zelensky is urging global leaders to provide defense assistance to Ukraine and help protect its airspace from Russia.

In a statement that comes amid Russia’s wide-ranging attack on Ukraine that began early Thursday, Zelenskyy said that Russia “has unleashed a war with Ukraine and the entire democratic world.”

He asked for world leaders to provide large-scale defense support and to protect Ukraine’s airspace from the “aggressor.”

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.

President Joe Biden pledged new sanctions to punish Russia for the aggression that the international community had expected for weeks but could not prevent through diplomacy.

Putin justified it all in a televised address, asserting that the attack was needed to protect civilians in eastern Ukraine — a false claim the U.S. had predicted he would make as a pretext for an invasion. He accused the U.S. and its allies of ignoring Russia’s demands to prevent Ukraine from joining NATO and for security guarantees. He also claimed that Russia does not intend to occupy Ukraine but will move to “demilitarize” it and bring those who committed crimes to justice.

Biden in a written statement condemned the “unprovoked and unjustified attack,” and he promised that the U.S. and its allies would “hold Russia accountable.” The president said he planned to speak to Americans on Thursday after a meeting of the Group of Seven leaders. More sanctions against Russia were expected to be announced Thursday.

Ukraine’s Foreign Minister Dmytro Kuleba described the assault as a “full-scale invasion” and said Ukraine will “defend itself and will win. The world can and must stop Putin. The time to act is now.”

In the capital, Kyiv Mayor Vitaly Klitschko advised residents to stay home unless they are involved in critical work and urged them to prepare go-bags with necessities and documents if they need to evacuate. An Associated Press photographer in Mariupol reported hearing explosions and seeing dozens of people with suitcases heading for their cars to leave the city.

“We are facing a war and horror. What could be worse?” 64-year-old Liudmila Gireyeva said in Kyiv. She planned to head to the western city of Lviv and then to try to move to Poland to join her daughter. Putin “will be damned by history, and Ukrainians are damning him.”

The Russian claims about knocking out Ukrainian air defenses and Ukrainian claims to have shot down several Russian aircraft could not immediately be verified. The Ukrainian air defense system and air force date back to the Soviet era and are dwarfed by Russia’s massive air power and its inventory of precision weapons.

The Russian Defense Ministry said it was not targeting cities, but using precision weapons and claimed that “there is no threat to civilian population.”

Anton Gerashchenko, an adviser to Ukraine’s interior minister, said on Facebook that the Russian military had launched missile strikes on Ukrainian military command facilities, air bases and military depots in Kyiv, Kharkiv and Dnipro.

After the initial explosions in Kyiv, people could be heard shouting in the streets. Then a sense of normality returned, with cars circulating and people walking in the streets as a pre-dawn commute appeared to start in relative calm.

The consequences of the conflict and resulting sanctions on Russia could reverberate throughout the world, upending geopolitical dynamics in Europe as well as affecting energy supplies in Europe and jolting global financial markets.