On Moscow's Red Square, a stage with giant video screens has been set up, with billboards proclaiming "Donetsk, Luhansk, Zaporizhzhia, Kherson - Russia!"
The head of the upper house of the Russian parliament said it could consider the incorporation of the four partially occupied regions on October 4, three days before President Vladimir Putin's 70th birthday.
The Russian-installed administrations of the four provinces have formally asked Putin to incorporate them into Russia, which Russian officials have suggested is a formality.
"This should happen within a week," Rodion Miroshnik, the Russia-installed ambassador to Moscow of the self-proclaimed Luhansk People's Republic, told the RIA state news agency
"The main thing has already happened – the referendum has taken place. Therefore, let's say: the locomotive has already started and it's unlikely to be stopped."
To annex the territories, which represent about 15 per cent of Ukraine, some sort of treaty will need to be struck and ratified by the Russian parliament, which is controlled by Putin allies. The areas will then be seen as part of Russia and its nuclear umbrella will extend to them.
Putin has warned he would use nuclear weapons to protect Russian territory from attack.
Residents who escaped to Ukrainian-held areas in recent days have told of people being forced to mark ballots in the street by roving officials at gunpoint. Footage filmed during the exercise showed Russian-installed officials taking ballot boxes from house to house with armed men in tow.
"They can announce anything they want. Nobody voted in the referendum except a few people who switched sides. They went from house to house but nobody came out," said Lyubomir Boyko, 43, from Golo Pristan, a village in Russian-occupied Kherson province.
Russia says voting was voluntary, in line with international law, and that turnout was high.
Ukrainian President Volodymyr Zelenskiy sought to rally international support against annexation in a series of calls with foreign leaders, including those of Britain, Canada, Germany and Turkey.
"Thank you all for your clear and unequivocal support. Thank you all for understanding our position," Zelenskiy said in a late-night video address.
The US has unveiled a $US1.1 billion ($A1.7 billion) weapons package for Ukraine that includes 18 High Mobility Artillery Rocket System (HIMARS) launchers, accompanying munitions, various types of counter drone systems and radar systems. The announcement brings the US security aid to $US16.2 billion.
The United States has also said it would also impose new sanctions on Russia for the referendums and the European Union's executive proposed more sanctions, but the bloc's 27 member countries will need to overcome their own differences to implement them.
Russia has announced it will mobilise some 300,000 reservists to bolster its forces in Ukraine. The conscription drive has sent thousands of Russian men fleeing to other countries.
On the ground, Ukraine and Russian forces are engaged in heavy fighting, especially in the Donetsk region, where the governor said six civilians were killed in Russian attacks on Wednesday.
Over the past 24 hours, Russia launched three missile and eight air strikes, carried out more than 82 attacks from rocket salvo systems on military and civilian sites, Ukraine's military said early on Thursday.
Ukraine's air force carried out 16 strikes on Wednesday, damaging or destroying a number of Russian positions, while ground forces destroyed two command posts, it said.
Valentyn Reznichenko, governor of the Dnipropetrovsk region, said three people were killed in Russian shelling of Dnipro, the region's capital, including a 12-year-old girl.
