Boris Johnson said Russian President Vladimir Putin’s invasion of Ukraine is a “catastrophe for our continent”, as he called on world leaders to meet and plan a response.

The Prime Minister tweeted his remarks after chairing an urgent Cobra emergency committee meeting on Thursday morning.

He is due to address the nation later, and speak in the Commons in the evening.

The Prime Minister said: “This is a catastrophe for our continent.

“I will make an address to the nation this morning on the Russian invasion of Ukraine.

“I will also speak to fellow G7 leaders and I am calling for an urgent meeting of all Nato leaders as soon as possible.”

Foreign Secretary Liz Truss summoned Russian ambassador to the UK Andrei Kelin, for the second time this week, to explain the “illegal, unprovoked invasion of Ukraine”.

Ms Truss said: “We will be imposing severe sanctions and rallying countries in support of Ukraine.”

It came as Ukraine President Volodymyr Zelensky declared martial law and urged people to stay indoors, amid reports of missile strikes and explosions across the country.

Ursula von der Leyen, president of the European Commission, accused Mr Putin of “bringing war back to Europe”.

Mr Zelensky called on Ukrainian nationals to volunteer for the resistance effort.

He said: “We will give weapons to anyone who wants to defend the country. Be ready to support Ukraine in the squares of our cities.

“We will lift sanctions on all citizens of Ukraine who are ready to defend our country as part of territorial defence with weapons in hands.

“We have severed diplomatic relations with Russia. For all those who have not yet lost their conscience in Russia, it is time to go out and protest against the war with Ukraine.”

Russia said it has only targeted Ukrainian air bases and other military assets, not populated areas.

But in a televised address, Mr Putin warned other countries that any attempt to interfere with the Russian action would lead to “consequences they have never seen”.

He said Russia does not intend to occupy Ukraine, and claimed responsibility for bloodshed lies with the Ukrainian “regime”.

Foreign Office minister James Cleverly described the comments as “bully” tactics and said the UK will respond with “unprecedented” sanctions “to punish the appalling decision” by Russia’s leader.

A Downing Street spokesman said Mr Johnson had assured Mr Zelensky during an early-morning telephone call that the West will “not stand by as President Putin waged his campaign against the Ukrainian people”.

Explosions could be heard in the Ukrainian capital, Kyiv, shortly after Mr Putin’s address, while explosions were also reported in the cities of Odesa and Kharkiv.

Elsewhere, footage appeared to show queues of people fleeing their homes, and Russian military crossing the border into Ukraine.

Mr Putin said the action was a response to threats from Ukraine.

And in a warning to the international community during his address on Russian television, he said: “Whoever tries to impede us, let alone create threats for our country and its people, must know that the Russian response will be immediate and lead to the consequences you have never seen in history.”

Foreign Office minister Mr Cleverly rubbished his explanation.

He told BBC Breakfast: “If Vladimir Putin thinks that he can scare the international community away from supporting Ukrainians in defence of their homeland, he is absolutely wrong on that and should be under absolutely no illusion that we will continue to support the Ukrainian government and Ukrainian people.”

In other developments:

– Transport Secretary Grant Shapps instructed the Civil Aviation Authority to ensure airlines avoid Ukraine airspace “following the horrific events overnight”.

– The UK’s ambassador in Ukraine, Melinda Simmons, repeated a call for Britons to leave the country.

– Leaders of the G7 nations – the UK, US, France, Germany, Italy, Canada and Japan – will have a virtual meeting to consider their response.

– Tom Tugendhat, chairman of the Foreign Affairs Committee, condemned Russia’s “vile act of war” against Ukraine, and also criticised the UK Government for imposing “weak” sanctions on Monday.

– European Commission president Ms von der Leyen said a package of massive and targeted sanctions will be presented to European leaders for approval.

– Airlines have begun suspending flights to and from Ukraine.

Former Cabinet minister David Davis called for Nato allies to provide air support to the Ukrainian army to assist the fight against Russian invaders while imposing the “most ferocious sanctions” against Moscow.

The senior Tory MP tweeted: “If Nato does not act now, Ukraine will be defeated in a matter of days.

“Therefore if we do not provide military support, more than 40 million Ukrainians will go from living in a democracy to living under a brutal dictatorship.

“It is far too late to get boots on the ground but it is not too late to provide air support to the Ukrainian army which may neutralise Putin’s overwhelming armoured superiority.”

The UK Foreign Office advised British nationals to leave Ukraine “immediately if you judge it is safe to do so”.

It added: “If you cannot leave safely, you should stay indoors, away from windows, and remain alert to developments that would allow you to leave safely.”