President Vladimir Putin on Monday hailed the "return" to Russia of annexed Ukrainian territories, after winning an election slammed as illegitimate by Western powers.

The ex-spy won over 87 percent of the vote in a three-day ballot that included voting in parts of Ukraine held by Russian forces.

Moscow has presented the weekend presidential election as proof that Russians have rallied around Putin more than two years into the Ukraine offensive.

Putin's victory is widely expected to further tighten his grip on Russia, where dissent is no longer tolerated under fast-accelerating repression.

In power since the last day of 1999, he is now on course to become the longest-serving Russian leader in more than two centuries.

"Hand in hand, we will move forwards and this will make us stronger... Long live Russia!" Putin told a crowd at a Red Square pop concert to mark 10 years since Russia's annexation of the Crimean peninsula from Ukraine.

Putin boasted of a new rail link in areas of Ukraine captured by Russian forces, saying those regions had "declared their desire to return to their native family".

He appeared at the concert alongside the three candidates who ran against him after hosting them at a Kremlin meeting in which they all congratulated him.

All of the 71-year-old's major opponents are dead, in prison or in exile, and voting took place a month after Putin's main challenger Alexei Navalny died in prison.

Authorities had called on Russians to take part in the vote out of patriotic duty.

"Vladimir Vladimirovich is the foundation of our country," said Viktoria, a 23-year-old IT worker at a state company, as she headed to the Red Square concert.

Elena, a 64-year-old economist, said she was not surprised by the result "because I think that any citizen who respects our country voted for Putin".

- Zelensky aid plea -

The three-day vote -- also held in occupied Ukraine -- was marred by spoiled ballots and Ukrainian bombardments.

Thousands responded to the opposition's call to protest against the election by forming long queues at polling stations -- both inside and outside Russia.

Yulia Navalnaya, who has vowed to continue her late husband Alexei's work, queued with crowds in Berlin on Sunday and said she had written his name on her ballot paper.

Moscow had warned Russians not to take part in the protests and on Monday dismissed the opposition.

"There are many people who... have completely broken away from the motherland," Kremlin spokesman Dmitry Peskov told reporters.

"Yulia Navalnaya, whom you mentioned, belongs to this group of people who lose their roots," he added.

Ukraine's President Volodymyr Zelensky, who had already slammed Putin for seeking to "rule forever", urged the US Congress on Monday to quickly approve a $60 billion military aid package.

"It is critically important for us that the Congress soon completes all the necessary procedures and makes a final decision... that will strengthen the Ukrainian economy and our armed forces," Zelensky said in a statement after meeting with US Senator Lindsey Graham.

His comments came as Ukraine has ramped up its attacks on Russian territory in recent days, with Ukrainian shelling killing four people near Russia's border city of Belgorod on Monday, bringing the total number of dead in the region since last week to 15, authorities said.

- 'That's life' -

Putin on Sunday said Navalny's name for the first time in public, breaking his years-long tradition of never referring to his opponent by name.

It was the first time he had commented on Navalny's death in prison on February 16.

Putin said that he had green-lighted an initiative for a prisoner swap including Navalny for Russians held in Western jails -- confirming allegations made by Navalny's team.

"I agreed on one condition: for us to exchange him and for him not to return," Putin said.

He said Navalny died days later.

"But this happens. There is nothing that you can do about it. That's life."

He did not say how Navalny died.

Navalny's team alleges that he was killed on the eve of a prisoner swap.

Navalny is the latest Putin opponent to die in mysterious circumstances.

- West slams vote -

The Kremlin said Putin held phone calls with his ex-Soviet allies in Central Asia, Belarus and Azerbaijan after the vote, as well as with Turkey's Recep Tayyip Erdogan, who congratulated his counterpart and offered to mediate between Moscow and Ukraine.

Putin also received congratulations from countries such as China, North Korea, Venezuela and Myanmar, Russian state media said.

But the result was met with scathing statements from Western leaders -- in contrast with the previous four elections Putin has won since 2000.

"This election has been based on repression and intimidation," the EU's foreign minister Josep Borrell said.

The UK also slammed the vote as unfair.

"Putin removes his political opponents, controls the media, and then crowns himself the winner. This is not democracy," Britain's foreign minister David Cameron said in a statement.