French President Francois Hollande has warned of "the risk for our country's future" if far-right leader Marine Le Pen wins the presidential election.
Speaking to voters from the Elysee Palace, the Socialist leader said France is in danger of "becoming isolated and breaking away from the EU" if Ms Le Pen sweeps to power.
He has endorsed Emmanuel Macron, who served as his economy minister from 2014 to 2016, as his successor.
Mr Hollande is the most unpopular president in France's history and did not seek re-election for himself.
His party's candidate, Benoit Hamon, was knocked out in the first round of the presidential election on Sunday after receiving just 6% of the vote.
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During his address, Mr Hollande claimed the purchasing power of the French people would be directly hit if Ms Le Pen wins the run-off vote on 7 May - with "unprecedented price increases" in stores and thousands of jobs being lost.
He added: "Faced with the terrorism threat - that demands solidarity and the cohesion of our country - the far-right would deeply divide France and would stigmatise a part of our citizens due to their origins or their religion.
"It would threaten our freedom and the principles that founded the Republic."
Mr Macron's independent En Marche! group - which translates into English as On The Move - won 23.75% of votes in the first round, while Ms Le Pen's Front National won 21.53%.
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Both candidates have now returned to the campaign trail, with less than two weeks to go before French voters return to the ballot boxes.
Ms Le Pen launched the first broadside of the run-off campaign, claiming her opponent is "weak" in the face of Islamist terrorism during a walkabout in Rouvroy.
In a move which appears to be aimed at appealing to a wider range of potential voters, Ms Le Pen has announced she is temporarily stepping down as Front National Leader to focus on her campaign.
The candidate said she wants to be "above partisan considerations" - with allies calling on those who backed the unsuccessful far-left candidate Jean-Luc Melenchon to continue voting "outside the system" by switching their support to Ms Le Pen.
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Meanwhile, Mr Macron's party spokesman has challenged Ms Le Pen on her claims she is the anti-establishment candidate who is taking on the country's elites.
Benjamin Griveaux said: "She's been in the political system for 30 years. She inherited her father's party and we will undoubtedly have Le Pens running for the next 20 years.
"So she is in a truly bad position to be talking about the elites and the people."
Mr Macron and Ms Le Pen will take part in a televised debate on 3 May - four days before the run-off vote, according to the AFP news agency.
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The FTSE 100 has enjoyed its biggest one-day rally since September as markets surged following the first round of voting.
London's leading share index rose by 2.11%, or 150 points, adding £38bn to the value of its constituent companies.
France's CAC 40 was lifted by 4%, while the DAX in Germany added more than 3% to take it to an all-time high.