In a morning address to the nation, Vladimir Putin announced a "partial mobilisation" of the Russian military and threatened the use of nuclear forces over Ukraine.
Shortly afterwards, flights sold out as Russians tried to flee the country to avoid Russia’s first military call up since the Second World War.
Speaking in his first nationwide address since he launched his "special military operation" in Ukraine on February 24, President Putin accused the west of "nuclear blackmail".
"I want to remind you that our country also has various means of destruction, and more modern than those of NATO countries," Putin said, adding "when the territorial integrity of our country is threatened, to protect Russia and our people, we will certainly use all the means at our disposal".
"This is not a bluff," he warned.
A senior Ukrainian military intelligence general told ITV News that Ukraine believes Putin is seriously considering nuclear war.
“It is possible Russia will use nuclear weapons against Ukraine. They will try to stop our offensive activity and to destroy our state,” Vadym Skibitskyi, Deputy Head of Ukraine’s Defence Intelligence told ITV News.
“This is a threat for other countries. The blast of a tactical nuclear weapon will have an impact not only in Ukraine but the Black Sea region,” Skibitskyi said, adding that the fallout from use of a nuclear weapon could affect countries including Poland, Turkey and Bulgaria
“We remember the history of Chernobyl. It is possible that we will have another catastrophe in our territory, in eastern Europe.”
In his speech, the president also backed a plan to annex large swathes of Ukrainian territory, currently occupied by Russia, which are roughly the size of Hungary.
He said that if the Donetsk and Luhansk regions in eastern Ukraine and Kherson and Zaporizhzhia in the south vote to "join" Russia in referenda planned for 23-27 September, Russia would consider them to be sovereign territory. In this case, Mr Putin indicated he would use full force to defend them.
The votes have been widely condemned by the west as a "sham" with the UK and US saying they will never recognise Russia's claims. When he launched Russia's "special military operation "on February 24, President Putin said it was "not our plan to occupy Ukrainian territory".
At the same time as announcing annexation plans and threatening nuclear force, the Russian president also said he had signed a decree ordering the "partial mobilisation" of the Russian military.
Following Putin's speech, Defence Minister Sergey Shoigu said Russia would start conscripting 300,000 reservists into the army.
The announcement triggered panic among some Russians. Tickets to countries which still have direct flights with Russia reportedly sold out as people scrambled to leave the country. There are no available flights listed in the next two days from Moscow to Istanbul in Turkey, nor Yerevan, the capital of Armenia.
Speaking this morning, Defence Minister Shoigu said Russia has the capacity to draw on 25 million people who have military experience and combat expertise but, for now, would be drawing on 300,000 reservists, "1.1% of the total mobilisation resource".
"This is not about some people who've never seen or heard about the army. This is about those who first of all served in the army and, secondly, have military-occupational expertise needed today in the Armed Forces and combat experience," Defence Minister Shoigu said, stressing that university students would not be called up.
Ukraine’s Defence Intelligence is preparing for some of the Russians called up to be deployed in the country over the coming months.
“Our assessment is that a new offensive is possible during winter and spring,” Skibitskyi said. “This is why Russia needs new units. The main goal of Russia is the occupation of all of Ukraine’s territory.”
Russia already operates national service, a peacetime conscription where all men aged 18-27, with exceptions for health reasons or education, complete a 12-month draft.
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Speaking on condition of anonymity, one Russian man told ITV News he was afraid he could be called up based on the fact he was conscripted into the military after finishing his university studies.
"They might call me up because I fall into the categories listed [by the defence minister] but the only thing is I have no fighting experience. There is nothing good about this," he said.
Another Russian said they were concerned that the phrasing of the mobilisation order was "obscure" and that the Russian military could use President Putin's order to "call up almost anyone".
Compared to Ukraine, life in Russia has, largely, continued unchanged but President Putin’s mobilisation announcement will bring the war much closer to ordinary Russians.
“People here are drinking in bars, dancing, they are happy and life goes on as normal,” one woman in St Petersburg told ITV News.
“Today everyone is reeling. They are shocked. Now there’s the feeling that everything isn’t okay. In the last few months people have totally forgotten and got used to what is happening. Now everyone is seriously shaking."
Following Putin's address, Russia's most well known opposition leader Alexey Navalny accused the president of throwing more Russians into the "meat-grinder."
Speaking in court via videolink from a penal colony where he is serving a prison sentence, Mr Navalny said: "It is clear that the criminal war is getting worse, deepening, and Putin is trying to involve as many people as possible in this” “He wants to smear hundreds of thousands of people in this blood."
Following Putin’s announcement, there were reports of a small number of protesters being detained in several cities across Russia including in Moscow, St Petersburg and Siberia.
There have been no widespread protests in Russia since the country introduced a law threatening anyone spreading “fake” information about the Russian military with a 15 year prison sentence.
President Putin launched his invasion in Ukraine with the aim of "denazifying" and "dimilitarising" the country. More than six months since the war started, Russian forces have failed in their attempts to take the Ukrainian capital Kyiv and have suffered a series of military defeats as the Ukrainian army has retaken thousands of kilometres of territory in the south and east of the country.
The performance of the military has been called into question within Russia, with the president's loyalists, including Chechen leader Ramzan Kadyrov, calling it "astounding," saying the military have "made mistakes". The latest announcement on "partial mobilisation" will likely cause more Russians to wonder whether their country's war in Ukraine is going to plan.
Western officials have said repeatedly that morale among Russian troops is low and there are widespread reports that Russia is trying to boost its fighting forces, including by recruiting prisoners.
In a recent video published online, a man believed to be Evgeny Prigozhin, a Putin ally who controls the Russian paramilitary organisation Wagner, is reportedly seen offering prisoners in a penal colony their freedom in exchange for six months military service in Ukraine.
“Nobody goes back behind bars,” he told them. “If you serve six months [in Wagner], you are free. If you arrive in Ukraine and decide it’s not for you, we will execute you.”
In an order published on the Kremlin's website today, Russia says anyone called up into the army should be paid at the same rate as a contract soldier. Even so, many Russians appear to be trying to avoid being drafted.
"I have no desire to fight," one Russian man told ITV News following Putin's announcement.
"The recent victories of the Ukrainian Army have forced Putin and the Russian military to find more and more human resources from across Russia," Valentyn Nalyvaichenko, former head of the Ukrainian Security Services and member of the Ukrainian Parliament, told ITV News.
"This partial mobilisation is an attempt by Putin to explain by propaganda why he is failing in Ukraine."
Since the start of the war, all independent media in Russia has closed and Russian journalists and opposition activists have fled abroad while others have been imprisoned.
Russians are largely unaware of the full scale of the war in Ukraine as state media depicts the conflict as a limited operation, mostly confined to the south and east of the country. Today Defence Minister Shoigu told Russians they had lost 5,937 soldiers in Ukraine, while western officials estimate the number is at least 15,000.
Paving the way for mobilisation, yesterday the Russian parliament introduced new amendments to the Russian criminal code which threaten anyone surrendering, deserting or refusing to fight with a prison sentence of up to 15 years.
Some Russians, anticipating being called up, booked flights to leave the country and left just minutes before Mr Putin gave his speech.
With prices for available flights now reaching several thousand pounds and travel made difficult by sanctions imposed on Russia for invading Ukraine, most Russians have no choice but to stay.
"I am not going to do anything,” a Russian man told ITV News when asked about the call-up.
“The choice is the army or a prison sentence. I hope they won't need me but I have nowhere to run to. It would be better if there were no war."