Russian President Vladimir Putin says Russia will carry out a special military operation in Donbrass, in south-eastern Ukraine.
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 soliders 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.
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.