The five leading candidates in the French election are appearing in the first TV debate of its kind.
They have been clashing over policing, the age of criminal responsibility and immigration.
Recent polls show Marine Le Pen of the far-right Front National getting most votes in the first round of voting.
But they also show centrist Emmanuel Macron or scandal-hit centre-right candidate Francois Fillon defeating her in a second round run off.
Voters go to the polls on 23 April. A second round will be held on 7 May if necessary.
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In his opening remarks, Mr Fillon said that if elected, he would be the president of what he called the “national recovery”.
Mr Macron said he would change the country’s traditional political divisiveness, while Ms Le Pen said she wanted a France that was not a “vague region” of the EU or subservient to Chancellor Angela Merkel’s Germany.
The BBC’s Paris Correspondent Lucy Williamson says Mr Macron – who has never fought an election before – has the most to lose in the debate and will probably be attacked by his rivals.
He will be hoping to show he can do the job.
Ms Le Pen, meanwhile, needs to boost her chances of winning an eventual second round while on the left, Mr Hamon is hoping to differentiate himself from Mr Melenchon, who is hoping to attract undecided voters, Le Parisien newspaper said.
Mr Fillon is looking to shake off the controversy surrounding payments to his wife for work she is alleged not to have carried out, and which are the subject of an investigation.