The move came after Russia held what it called referendums in occupied areas of Ukraine and declared the annexation of the Donetsk, Luhansk, Zaporizhzhia and Kherson regions.
Western governments and Kyiv said the votes breached international law and were coercive and unrepresentative, the Reuters news agency reported.
US President Joe Biden said ahead of the Moscow ceremony that Washington would never recognise Russia's claims on Ukraine's sovereign territory and vowed to slap new sanctions on Moscow over its annexation of new areas of Ukraine.
UK Prime Minister Liz Truss said on Friday that Britain would never accept the regions of Donetsk, Luhansk, Kherson and Zaporizhzhia as “anything other than Ukrainian territory.”
European Commission chief Ursula von der Leyen said that Russia's annexation of the four Ukrainian regions was illegal and that occupied land would remain part of Ukraine.
"The illegal annexation proclaimed by (Russian President Vladimir) Putin won't change anything," von der Leyen said in a tweet.
"All territories illegally occupied by Russian invaders are Ukrainian land and will always be part of this sovereign nation," she added.
Polish Deputy Foreign Minister Szymon Szynkowski vel Sęk said in a media interview that "pseudo-referenda" held by Russian authorities in occupied parts of Ukraine’s Kherson, Zaporizhzhia, Donetsk and Luhansk regions had been designed "to justify the incorporation of occupied territories" into Russia.
"The international community will never accept it,” Szynkowski vel Sęk added.
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Source: IAR, PAP, Reuters