Vladimir Putin has vowed to fire back with a "severe" response to any attacks on Russia after unleashing a barrage of missiles on cities across Ukraine, including the capital Kyiv, on Monday.
The Russian President ordered 83 missiles to be fired at Ukraine, of which half were shot down, in a revenge attack following the partial destruction of a key bridge in Crimea over the weekend.
As explosions rocked Kyiv, more blasts were felt in Dnipro, Zaporizhzhia, Kharkiv, Sumy, Zhytomyr, Vinnytsia, and even Ternopil and Lviv in the west of the country.
Eleven people were killed, with more than 60 injured.
Dmitry Medvedev, Putin's henchman and former president, warned it was just the "first episode" of a major retaliation.
And Belarus dictator Alexander Lukashenko announced that Russian troops will deploy with his forces after accusing Ukraine, Poland and Lithuania of "preparing attacks" against him. Belarus is closely allied with Russia but before now was yet to join in military action against Ukraine.
"Strikes on the territory of Belarus are not just being discussed in Ukraine today, but are also being planned," Lukashenko said after speaking with Putin on Monday. "Their owners are pushing them to start a war against Belarus to drag us there.
"We have been preparing for this for decades. If necessary, we will respond.
"Tell the president of Ukraine and the other lunatics: if they touch one metre of our territory then the Crimean Bridge will seem to them like a walk in the park."
Putin also spoke hours after Monday's blistering attack, accusing Ukraine of "carrying out terrorist acts" on Russian territory and saying his military would continue to respond "proportionately".
"A massive strike with long-range high-precision weapons, from air, sea and land against energy infrastructure, communications and military targets was carried out this morning at the suggestion of the Defence Ministry and the General Staff.
"Should the attempts to carry out terrorist attacks on our territory persist, Russia's response will be tough and its scale will be proportionate to the level of threats posed against Russia.
"Kyiv's regime, with its actions, places itself in line with international terrorist organisations," Putin said. "Leaving such crimes without response is impossible. In case of continuing attacks we will respond in harsh manner and in line with level of threats to Russian Federation. Nobody should have any doubt about this."
Ukrainian President Volodymyr Zelenskyy said Russia was targeting energy infrastructure - and civilians.
"They want panic and chaos, they want to destroy our energy system. They are hopeless.
"Such a time and such targets were specially chosen to cause as much damage as possible.
"Russia is trying to destroy us and wipe us off the face of the earth."
Buildings - including the empty German consulate in Kyiv - and cars were left ablaze in the wake of the attack.
And in an eerie similarity to the Crimea bridge explosion, the Klitschko bridge, which opened in 2019 and is popular with walkers and cyclists, was seen engulfed in a fireball as a missile struck at around 8am local time.
The 212-metre bridge is normally packed with pedestrians and street performers, with a motorway running underneath. It is not clear if anyone was killed or hurt in the explosion.
Loud blasts were heard in the centre of Kyiv early on Monday and graphic videos show missiles exploding metres away from civilians.
The explosions took place around 8am local time, with air raid sirens sounding in the Ukrainian capital more than an hour before the blasts.
Videos posted on social media showed black smoke rising above several areas in the city.
The strikes are a worrying escalation from Russia's military, which has not targeted the city since June 26. Ukraine's presidency confirmed there were strikes on "many" cities in Ukraine and not just the capital.
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"A girl was recording herself as she walked through what looks like Shevchenko Park in Kyiv this morning. She was almost killed by a Russian rocket," Wall Street Journal reporter Matthew Luxmoore tweeted.
"Several missile strikes in the center of Kyiv. Burning cars with people who were commuting to work. The strike was deliberately carried out during rush hour on a busy street to inflict maximum losses and intimidate. A pure act of terrorism," security expert Maria Adeeva said.
Analysts say Putin has timed the strikes to "maximise civilian causalities".
"(This is) the biggest series of salvoes since the war began. Putin's rage is unlikely to significantly weaken Ukraine's military ability, but it will increase Ukrainians' motivation to fight," foreign affairs expert Yaroslav Trofimov tweeted.