Ukraine has said it will seek to take back its territory.
“Referendums have no legal value, under international law the regions are and remain territories of Ukraine and Ukraine is ready to do anything to take them back,” Mikhailo Podolyak, Zelensky’s adviser, told Italian newspaper La Repubblica.
“They were sham votes, in which few people participated. To those who went to vote they pointed their rifles in their faces ordering, ‘Vote!’.”
Mr Zelensky promised a strong response to the annexations and summoned his defence and security chiefs for an emergency meeting on Friday, where “fundamental decisions” would be taken, an official said.
Ahead of the annexation ceremony in the Georgievsky Hall of the Great Kremlin Palace and a concert in Red Square, Mr Putin said “all mistakes” made in the call-up announced last week should be corrected, his first public acknowledgment that it had not gone smoothly.
Thousands of men have fled Russia to avoid a draft billed as the enlistment of those with military experience and required specialities, but has often appeared oblivious to individuals’ service record, health, student status or even age.
At Friday’s event, Putin was scheduled to give a speech, meet leaders of the self-styled Russian-backed Donetsk People’s Republic and Luhansk People’s Republic, and to meet as well with Moscow-installed leaders of the parts of Kherson and Zaporizhzhia that Russian forces occupy.
US President Joe Biden said the United States would never recognise Russia’s claims on Ukraine’s territory, denouncing the referendums.
“The results were manufactured in Moscow,” Mr Biden said at a conference of Pacific Island leaders on Thursday.
Turkish President Tayyip Erdogan pressed Putin in a call to take steps to reduce tensions in Ukraine.
Mr Guterres told reporters: “Any decision to proceed with the annexation would have no legal value and deserves to be condemned.”
The US and the European Union are set to impose additional sanctions on Russia over the annexation, and even some of Russia’s close traditional allies, such as Serbia and Kazakhstan, say they will not recognise the take-over.
What Russia was billing as a celebration comes after it has faced its worst setbacks of the seven-month-old war, with its forces routed in Ukraine’s northeast Kharkiv region.
Heavy fighting continues in the four disputed regions.
Some military experts say Kyiv is poised to deliver another major defeat, gradually encircling the town of Lyman, Russia’s main remaining bastion in the northern part of Donetsk province. Its capture would open the way into the Luhansk region.
In the past 24 hours, Ukraine’s military said it had killed 43 Russian servicemen and destroyed two Russian Su-25 aircraft, four tanks, four large-calibre howitzers, two self-propelled cannons and one reconnaissance drone.