Boris Johnson has vowed that Britain “cannot and will not just look away” from Vladimir Putin’s invasion of Ukraine and pledged to unite with allies to respond with a massive package of sanctions designed to “hobble the Russian economy”.

In a televised statement at midday on Thursday, the Prime Minister said the world cannot stand by and allow the freedom of Ukraine to be “snuffed out”, as Moscow hit its neighbour with a wide-ranging attack, targeting cities and bases with air strikes or shelling.

“This act of wanton and reckless aggression is an attack not just on Ukraine, it’s an attack on democracy and freedom in eastern Europe and around the world,” Mr Johnson said from Downing Street.

The Prime Minister criticised the Russian president for having “unleashed war in our European continent”, attacking Ukraine “without any provocation and without any credible excuse”.

Mr Johnson, who chaired an emergency meeting of the Cobra committee on Thursday morning, said: “Innumerable missiles and bombs have been raining down on an entirely innocent population.

“A vast invasion is under way by land, by sea and by air.

“We, and the world, cannot allow that freedom just to be snuffed out. We cannot and will not just look away.

“Today in concert with our allies we will agree a massive package of economic sanctions designed in time to hobble the Russian economy.

“Diplomatically, politically, economically, and eventually, militarily, this hideous and barbaric venture of Vladimir Putin must end in failure.”

Mr Johnson warned of the prospect of “grim” months to come before echoing an earlier address from Ukrainian President Volodymyr Zelensky in speaking directly to the Russian public.

“I cannot believe this is being done in your name or that you really want the pariah status it will bring to the Putin regime,” the Prime Minister said.

Foreign Secretary Liz Truss accused Moscow of an “egregious violation of international law” and said the UK and allies will respond with an “unprecedented package of further sanctions”.

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

Nato Secretary-General Jens Stoltenberg condemned the invasion as a “blatant violation” of international law and said he is calling a virtual summit of alliance leaders on Friday to discuss the “serious threat” to security in the region.

“This is a grave moment for the security of Europe. Russia’s unprovoked and unjustified attack on Ukraine is putting countless lives at risk,” he said from Nato’s headquarters in Brussels.

Nato agreed to strengthen its eastern flank, while European Commission president Ursula von der Leyen accused Mr Putin of “bringing war back to Europe”.

Mr Zelensky declared martial law in the early hours of Thursday and urged people to stay indoors as he called on Ukrainian nationals to volunteer for the resistance effort.

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

“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.”

The Kremlin has claimed it is only targeting Ukrainian air bases and other military assets, not populated areas, but Kyiv said at least 40 people have died so far.

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”.

Explosions could be heard in Kyiv moments later, while blasts 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.

In other developments:

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

– Mr Johnson will join US President Joe Biden and other G7 leaders to discuss their response to the crisis during a virtual meeting.

– 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.

– Airlines have begun suspending flights to and from Ukraine.

Mr Johnson held an early-morning telephone call with the Ukrainian president, telling him the West will “not stand by as President Putin waged his campaign against the Ukrainian people”.

The Prime Minister said every Briton’s thoughts are with the Ukrainian people “during this dark time” and raised hopes they can “resist” the Russian offensive.

Mr Johnson is scheduled to address MPs in the Commons on Thursday evening.