PARIS: The campaign for the first round of France's presidential election on April 22 officially kicked off Monday with candidates allowed to put up posters and given free air time on television.

Unofficial campaigning has already been underway for weeks, with incumbent Nicolas Sarkozy facing a tough challenge from Socialist frontrunner Francois Hollande.

As of Monday, the 10 registered candidates were allowed to put up campaign posters in areas designated by local authorities, although they were not expected to go up until Tuesday, after the Easter holiday.

The posters must be of the same size and not use a white background, which is reserved for official announcements, or any combination of the blue, white and red colours of the French flag.

Public television and radio stations will now be allowed to show campaign spots, with each candidate allowed a total of 43 minutes of air time until April 20.

All broadcasters have been required to provide equal air time to all candidates following their official registration on March 19.

Candidates will now also be allowed to mail campaign platforms to some 45 million registered voters, all restricted to the same size and format.

The campaign officially ends on April 21, with candidates barred from making public statements and no opinion polls allowed to be published.

Hollande is leading in the polls to win the May 6 second round but after months of trailing, Sarkozy has in recent weeks moved slightly ahead in first-round voting intentions.

The latest IFOP-Fiducial survey released Friday showed Sarkozy ahead with 29 per cent to Hollande's 26.5 per cent in the first round, though Hollande would win the run-off with 53 per cent.

Far-right candidate Marine Le Pen of the National Front was third with 16.5 per cent, followed by Left Front contender Jean-Luc Melenchon with 12.5 per cent and centrist Francois Bayrou with 10 per cent.