Russian President Vladimir Putin says Russia will carry out a special military operation in Donbrass, in south-eastern Ukraine.
Explosions have already been heard in Kyiv, Kharkiv and in the Belgorod province of Russia.
Explosions also rocked the breakaway eastern Ukrainian city of Donetsk and civilian aircraft were warned away as the United States said a major attack by Russia was imminent.
Putin made the declaration in a televised speech at the same time as the UN Security Council was imploring him to stop.
Putin says Russia does not plan to occupy Ukrainian territory.
He called on Ukrainian soldiers to immediately lay down their weapons and go home.
Putin says in case of foreign interference, Russia will react immediately.
He says the responsibility for any bloodshed will be on the conscience "of the Ukrainian regime", RIA News agency reports.
Russia's actions are self-defence from the threats and from bigger problems than those today, he said.
He said he wants to try demilitarise and 'de-Nazify' Ukraine.
Putin added that clashes between Ukrainian and Russian forces are "inevitable" and "only a question of time".
UN Secretary General Antonio Guterres has been speaking at the emergency meeting of the security council, issuing a direct plea to Russia's president.
He asked Putin to stop his troops from attacking Ukraine and urges him to give peace a chance.
A NATO official told Reuters it very much looked like the Russian invasion of Ukraine had begun.
A Reuters witness in Kyiv reported hearing a series of distant loud noises similar to the firing of artillery.
The BBC's Eastern Europe correspondent Sarah Rainsford, who is in the Donetsk region of eastern Ukraine, said he heard a loud explosion.