Russian President Vladimir Putin is due to stage a ceremony in an ornate Kremlin hall to proclaim Russia's rule over around 15% of Ukraine, the biggest annexation in Europe since the Nazi era.
The proclamation is to be followed by a celebratory pop concert outside the Kremlin walls on Red Square.
Russia's annexation of the Russian-occupied areas of Donetsk, Luhansk, Kherson and Zaporizhzhia has been condemned in the West and beyond.
UN chief Antonio Guterres called it a "dangerous escalation" and a violation of the United Nations charter.
"It can still be stopped. But to stop it we have to stop that person in Russia who wants war more than life. Your lives, citizens of Russia," Ukrainian President Volodymyr Zelenskyy said in a Thursday evening address.
The Russian annexation, held after what the West denounced as phoney referendums at gunpoint in occupied territory, followed weeks of defeats for Moscow's forces on the battlefield in which they were routed from their positions in Ukraine's northeast.
Putin has ordered the call-up of hundreds of thousands of Russian reservists, a move that prompted tens of thousands of Russian men to flee over borders to escape being shipped off to war.
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