France's President Emmanuel Macron is this week expected to launch his re-election bid and join a race now overshadowed by the war in Ukraine.

Other presidential candidates have been campaigning for weeks already, and it was an open secret that Macron would eventually enter the race and bid to win a second five-year term. Legally, all candidacies for the presidential election must be registered by Friday.

Recent opinion polls have shown that centrist Macron, who in 2017 became France's youngest-ever president, is likely to come out on top in the first round of the vote on April 10.

The outcome of a run-off vote two weeks later, however, is less clear, as potential candidates from far-right parties who have dominated the early stages of the campaign would present stronger opposition, reported the AFP news service.

Marine Le Pen and Eric Zemmour are vying for votes from the extreme right, followed by conservative candidate Valerie Pecresse. Polls show Jean-Luc Melenchon as the most popular candidate from the far-left.

Macron's campaign will focus on a few key policy ideas, sources told the Reuters news agency.

"Everything is ready for the first 15 days following the announcement of his candidacy: first campaign trip, which meeting, which media and themes," a government source said.

"We will focus on three or four major policy proposals on issues such as rebuilding France, a new social contract.

The idea is not just to prolong his mandate but to surprise, propose, shake things up," the source said, adding that Macron "should take part in a campaign rally in Marseille, another in Paris and a third one in western France".

The race has been shaken by the crisis in Ukraine, and the main candidates have come under attack over previous pro-Russia stances, reported The Guardian.

It said Russia's attack is expected to be a key issue in the presidential race political debate.

Le Pen and Zemmour have been criticized for trying to distance themselves from previous pro-Kremlin comments.

The paper noted that in 2014, Le Pen obtained a loan from a Russian bank to fund her 2017 campaign and that she made a high-profile visit to Russia at the time, when she praised President Vladimir Putin. She issued a statement last week, calling Russia's moves in Ukraine unjustified, and said they must be condemned "without ambiguity".

Previously, Zemmour had suggested that "Putin is not an aggressor", but was instead "being treated aggressively by the international community", noted AFP. Last week Zemmour said he "condemned without reservation "what he called Russia's "unjustifiable" military intervention.

Hard-left candidate Melenchon condemned the Russian action "as an extremely serious act of war "and "an unacceptable violation of the principles of international law".

Pecresse said the crisis in Ukraine was "a turning point" in the presidential campaign because it had allowed "masks to fall", noted The Guardian. She said Zemmour, Le Pen and Melenchon were all now "discredited".