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Ukraine is to impose a month-long state of emergency from Thursday after it put reserve forces on standby, a day after Russia formally recognised two breakaway enclaves in the east.
The Ukrainian government also announced compulsory military service for all men of fighting age.
The announcements were made as Ukraine's Parliament, Cabinet and Foreign Ministry websites went down. Government websites have experienced several cuts in recent weeks, which Kiev has blamed on cyber attacks.
President Volodymyr Zelenskyy has put Ukraine's more than 200,000 reservists on notice that they will receive a summons to return to their units.
Ukraine's Parliament overwhelmingly approved the national state of emergency on Wednesday.
Regional governments are to adopt greater security measures that range from tighter ID and vehicle checks to more stringent policing.
Kiev mayor Vitali Klitschko said that "mass events and strikes" would be banned across the country under the new rules.
He said specific measures for Kiev included checkpoints at the main entrances to the capital and "special controls" at train stations and airports.
"We must be ready to defend our homes, families, city and country at any moment," Mr Klitschko said.
Pro-Russian separatists have controlled parts of Donetsk and Luhansk since 2014. Russia recognised those regions as independent states and approved use of its troops abroad as "peacekeepers" this week.
Orysia Lutsevych, manager of the Ukraine Forum in the Russia and Eurasia Programme at Chatham House, told a briefing in London that the events started by Russian President Vladimir Putin had seen Ukrainians rally to the country's defence, but that it faces challenges with supplies.
“From my conversations with people in Ukraine, there's a deficit in buying firearms,” Ms Lutsevych said.
“Many people want to acquire them but the procedures and licensing for them have not been simplified and there's simply not enough available in the market, so it tells you the mood.
“The citizens pretty much are ready for action.
“Almost 23 per cent of Ukrainians are ready to join the armed forces. A lot are ready to volunteer. There will be resistance and it will not be a free ride for Putin."
She said there was a palpable sense of a backlash against the Kremlin.
“There is a massive switch of language on social media from Russian into Ukrainian,” Ms Lutsevych said.
Ms Zelenskyy, speaking alongside the visiting leaders of Poland and Lithuania, said: “We are united in believing that the future of European security is being decided right now, here in our home, in Ukraine.
“Ukraine needs security guarantees; clear, specific and immediate. I believe that Russia must be among those countries giving clear security guarantees.”
Western capitals say Moscow has amassed 150,000 troops in combat formations on Ukraine's borders with Russia, Belarus and Russian-occupied Crimea, and on warships in the Black Sea.
Ukraine also urged its about three million citizens living in Russia to leave.
Websites for Ukraine's ministries of foreign affairs, defence, and internal affairs, along with its security service and Cabinet, have just been affected by network disruptions.
The incident appears consistent with recent cyber attacks.