President Vladimir Putin's order to mobilise another 300,000 Russians to fight escalates a war that has already killed thousands, displaced millions and damaged the global economy.
The mass conscription may be the riskiest domestic move of Putin's two decades in power, after Kremlin promises it would not happen and a string of battlefield failures in Ukraine.
Anti-war protests in 38 Russian cities saw 1400 people arrested on Wednesday, a monitoring group said. Some had been served summons to report to enlistment offices on Thursday, the first full day of conscription, independent news outlets said.
Flights out of Russia sold out after Putin's announcement.
"Every normal person is (concerned), it's horrible," said one man, identifying himself only as Sergey, disembarking in Belgrade after a flight from Moscow.
"It is ok to be afraid of the war and such things."
Ukraine urged the United Nations to create a special tribunal and strip Moscow of its UN Security Council veto power as a diplomatic showdown loomed on Thursday in New York.
"A crime has been committed against Ukraine, and we demand just punishment," President Volodymyr Zelenskiy, dressed in his trademark green military T-shirt, told world leaders by video at the annual UN General Assembly on Wednesday.
The Security Council has been unable to take significant action on Ukraine because Russia is a permanent veto-wielding member, along with the United States, France, Britain and China.
Russian Foreign Minister Sergei Lavrov will face Ukrainian and Western counterparts when UN Secretary-General Antonio Guterres and International Criminal Court prosecutor Karim Khan brief the 15-member council later on Thursday.
French President Emmanuel Macron, who has spoken to Putin numerous times this year, said the goal remained for a negotiated peace despite the "mistake" of conscription.
On the ground, Russia's military fired nine missiles on the city of Zaporizhzhia, hitting a hotel and a power station, said regional governor Oleksandr Starukh.
At least one person was killed with others trapped under rubble, he said. Zaporizhzhia is about 50km from the nuclear plant of the same name.
Ukraine's armed forces said Russia had in the last 24 hours launched eight missile and 16 air strikes and fired 115 anti-aircraft missiles at military and civilian targets, mostly in the Zaporizhzhia and Donetsk areas in the south and east respectively.
Putin also announced moves to annex four Ukrainian provinces - about 15 per cent of Ukrainian territory - and threatened to use nuclear weapons to defend Russia, declaring: "It's not a bluff."
Western leaders, including US President Joe Biden, have assailed Putin this week as "reckless" and desperate due to a successful Ukraine counter-offensive in recent days.
Ukraine extended its hold on recaptured northeastern territory earlier this week as troops marched farther into areas abandoned by Russia, paving the way for a potential assault on occupation forces in the Donbas industrial heartland.
Russia and Ukraine carried out an unexpected prisoner swap on Wednesday, the largest since the war began and involving almost 300 people, including 10 foreigners and the commanders who led a prolonged Ukrainian defence of Mariupol earlier this year.
"We're now out of the danger zone and on our way home to our families," one of those whom Russian forces released, Britain's Aiden Aislin, said in a video from a plane posted on social media. "By the skin of our teeth," added also-released compatriot Shaun Pinner beside him.
Both men had been sentenced to death by a court in the Russian-backed breakaway region of Donetsk.
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