Right-wing presidential hopeful Nicolas Sarkozy has extended his lead over his main rival, Socialist Segolene Royal, but a poll on Sunday showed that nearly half of French voters were still undecided.
With just two weeks to the first round of the presidential election, the CSA poll for Le Parisien newspaper said 42 percent were unsure which way to vote.
The number is slightly higher than before previous presidential ballots and highlights the unpredictable nature of the forthcoming election.
"People are hesitating more and more between the candidates and will decide at the last moment," said Roland Cayrol, director of the CSA institute.
Although election sparring has been going on for months, Monday marks the official start of the campaign when the candidates will unveil media spots and stick up posters.
Sarkozy has consistently led opinion polls and has picked up a few points over his main rivals in recent days.
The tough former interior minister has benefited from a renewed focus on immigration and security issues and violence between youths and police at Paris' Gare du Nord train station.
An IFOP survey for Journal du Dimanche newspaper on Sunday scored Sarkozy at 29.5 points in the first round vote on April 22, a three and a half point rise from the previous IFOP poll.
Royal's first round position dropped three points to 22 and the survey showed that Sarkozy would comfortably beat her with 54 points to 46 in the second round on May 6.
Royal's campaign has been struggling, with members of her party complaining that she lacks a clear strategy.
She angered some Socialists last week with a plan to encourage small firms to hire school leavers, while rivals called it costly and unrealistic.
Sensing the election is wide open, the third and fourth placed candidates have stepped up attacks on the poll leaders.
Centrist Francois Bayrou, in third place, has sought to take advantage of Royal's weaknesses by talking about social issues and he has tried to win right-wing votes by criticising Sarkozy.
The former education minister has portrayed himself as an alternative to the pair and has climbed steadily in the polls since the beginning of the year.
He says he will be in the second round and polls show he would beat Sarkozy if they are both in the May 6 runoff.
Far-right leader Jean-Marie Le Pen, who shocked France by finishing second in the 2002 election, says he will also be in the second round and has even reserved a convention centre for that evening.
The 78-year-old is in fourth place but polls show his support has risen in the last week, boosted by the Gare du Nord clashes and concerns about immigration and security.
Güncelleme Tarihi: 20 Eylül 2018, 18:16