It came as Ukraine’s president declared martial law, while Russia’s military said it had targeted Ukrainian air bases and other military assets and had not targeted populated areas.
A Downing Street spokesperson said the Prime Minister will chair a Cobra committee meeting at 7.30am to discuss the response to the “horrific attacks”.
In a statement on Twitter, Mr Johnson said he had spoken with Ukrainian President Volodymyr Zelensky to discuss “next steps”.
“I am appalled by the horrific events in Ukraine and I have spoken to President Zelenskyy to discuss next steps,” he said.
“President Putin has chosen a path of bloodshed and destruction by launching this unprovoked attack on Ukraine.
“The UK and our allies will respond decisively.”
A Downing Street spokesperson said Mr Johnson had assured Mr Zelensky the West “would not stand by as President Putin waged his campaign against the Ukrainian people”.
“The Prime Minister said he hoped Ukraine could resist and that Ukraine and its people were in the thoughts of everyone in the United Kingdom people during this dark time,” the spokesperson said.
First Minister Nicola Sturgeon said the Kremlin “must face the severest consequences” for “unprovoked aggression” in Ukraine.
She tweeted: “Overnight developments in Ukraine – however anticipated – are appalling and horrific. The Kremlin must face the severest consequences for this unprovoked aggression. And the world must stand in solidarity with the people of Ukraine #StandWithUkraine.”
Mr Putin announced the action during a televised address early on Thursday morning, saying the move was a response to threats from Ukraine.
He said Russia does not have a goal to occupy Ukraine, but the responsibility for bloodshed lies with the Ukrainian “regime”.
He also warned other countries that any attempt to interfere with the Russian action would lead to “consequences they have never seen”.
Mr Putin accused the US and its allies of ignoring Russia’s demand to prevent Ukraine from joining Nato and offer Moscow security guarantees.
He said the Russian military operation aims to ensure a “demilitarisation” of Ukraine, adding that all Ukrainian servicemen who lay down arms will be able to safely leave the zone of combat.
Explosions could be heard in the Ukrainian capital of Kyiv shortly after Mr Putin’s address, while explosions were also reported in the cities of Odesa and Kharkiv.
Ukraine’s border guard agency said the Russian military attacked the country from neighbouring Belarus.
The agency said that the Russian troops unleashed an artillery barrage as part of an attack backed by Belarus. They said the Ukrainian border guards were firing back, adding that there was no immediate report of casualties.
The UK Foreign Office has said people should not attempt to cross from Russia into Ukraine. The official advice was updated on Thursday to say: “There are multiple reports of widespread military activity in Ukraine. You should not attempt to cross into Ukraine from Russia.”
Defence Secretary Ben Wallace described Russia’s actions as “naked aggression against a democratic country” and said no one had been fooled by the Kremlin’s “false flags and fake narratives”.
“The Russian Federation has today further violated Ukrainian Sovereignty,” he said.
“Despite the efforts of the international communities, Russia has chosen conflict. No one has been fooled by the Kremlin’s false flags and fake narratives.
“This is naked aggression against a democratic country which had dared to express a different aspiration than being a supine neighbour to Russia. No one should forget this day. Putin thinks this land grab is about securing his legacy – it will be, but not the one that he wishes.”
US President Joe Biden denounced Russia’s decision to conduct a military operation in Ukraine as an “unprovoked and unjustified attack”, declaring “the world will hold Russia accountable”.
Mr Biden said on Twitter: “President Zelenskyy reached out to me tonight and we just finished speaking. I condemned this unprovoked and unjustified attack by Russian military forces. I briefed him on the steps we are taking to rally international condemnation, including tonight at the UN Security Council.
“He asked me to call on the leaders of the world to speak out clearly against President Putin’s flagrant aggression, and to stand with the people of Ukraine.
“Tomorrow, I will be meeting with the Leaders of the G7, and the United States and our Allies and partners will be imposing severe sanctions on Russia. We will continue to provide support and assistance to Ukraine and the Ukrainian people.”
Britain’s ambassador to Ukraine urged UK nationals to leave Ukraine immediately.
Melinda Simmons wrote on Twitter: “We advise GB nationals to leave Ukraine immediately if you judge that it is safe to do so from your location. Please call +380 44 490 3660 if you need assistance.”
A British-Ukrainian student living in Kyiv said she woke up to the sounds of explosions on Thursday morning.
Valentina Butenko is studying politics at University College London, but returned to Ukraine a few weeks ago to offer help with the security crisis there.
“I woke up this morning to the sound of an explosion,” the 19-year-old, who has a British mother but was raised in Ukraine, told the PA news agency.
“I live on the outskirts of Kyiv so I expect it must have been pretty huge for me to hear it.
“Have been hearing explosions intermittently since 5am … the sky has lit up a few times although it seems to have calmed down slightly now.”
Ms Butenko said “a lot” of people are leaving the Ukrainian capital and roads out of the city are blocked, making it “impossible to get out”.