Russian President Vladimir Putin authorized "a special military operation" against Ukraine on Thursday morning to eliminate what he called a serious threat, saying his aim was to demilitarize Russia's southern neighbor.
In an early-morning address on state television, Putin said he had been left with no choice but to launch the operation, the scope of which was not immediately clear but appeared to go well beyond helping Russian-backed separatists in eastern Ukraine.
"I have decided to conduct a special military operation," said Putin, seated at a desk in the Kremlin next to a battery of telephones, with the Russian flag behind him.
"Its goal is to protect people who have been subjected to bullying and genocide... for the last eight years. And for this, we will strive for the demilitarization and denazification of Ukraine.
"And to bring to court those who committed numerous bloody crimes against civilians, including against citizens of the Russian Federation."
Ukraine dismisses as cut from whole cloth Russian accusations of genocide against people living in parts of its east seized by Russian-backed separatists in 2014. Kyiv has said Putin was looking for an artificial pretext to attack it.
The Kremlin chief's announcement followed an appeal from the Russian-backed separatists in eastern Ukraine for military help against what they said was growing Ukrainian aggression.
Kyiv has denied any such aggression.
About an hour and a half after the start of the invasion, Ukrainian President Volodymyr Zelensky imposed martial law all across the country. The country's airspace has been completely closed and Israeli airlines have canceled all flights into and out of the country.
Putin told the Ukrainian military to lay down its weapons and go home.
"I urge you to immediately lay down your weapons and go home. All servicemen of the Ukrainian army who fulfill this demand will be able to freely leave the combat zone and return to their families," said Putin who spoke with anger in his voice at times.
Explosions were heard in the capital Kyiv, in the eastern city of Kharkiv, in Odesa on the shores of the Black Sea and in Mariupol near the Donbas region which houses the two pro-Russian separatist provinces of Donetsk and Luhansk.
Ynet and Yedioth Ahronoth military correspondent Ron Ben-Yishai reported hearing loud blasts in Kyiv around 5am (local time).
"These are probably cruise missiles that were apparently launched to paralyze anti-aircraft batteries, military facilities and command and control facilities at airports that are located about 30 kilometers (20 miles) out of the city," he said.
"This is part of a larger Russian offensive on all parts of Ukraine."
The Russian army announced that it was an attack on military infrastructure using precision weapons and that no attack had been launched on the cities themselves.
"Russia cannot feel safe, develop, and exist with a constant threat emanating from the territory of modern Ukraine," Putin said.
Russia would respond instantly if any external force tried to interfere with its actions, he added.
"Whoever tries to hinder us, and even more so, to create threats to our country, to our people, should know that Russia's response will be immediate. And it will lead you to such consequences that you have never encountered in your history.
"We are ready for any development of events. All the necessary decisions have been made in this regard. I hope that I will be heard."
Putin's comments come after the United States said Russia had stationed nearly 150,000 troops near Ukraine's borders.
Putin repeated past complaints about the failure of NATO and the United States to satisfy Russia's security demands.
Ukrainian forces later reported they had also been attacked from Belarus in the north, not far from Kyiv itself.
CNN reported that these were Russian forces crossing the border there, after Russia had massed large military forces in Belarus in recent weeks, including tanks.
Authorities also reported an attack from the southern Crimean peninsula, which Russia annexed from Ukraine back in 2014.
Meanwhile, pro-Russian separatists in the Donbas region of eastern Ukraine also announced that they had launched an "extensive offensive" on the front with the Ukrainian army in the region.
Meanwhile, the UN Security Council convened an emergency hearing at Ukraine's request. G7 leaders are expected to hold a virtual meeting on Thursday to discuss the unfolding situation.