Socialist Francois Hollande has won the first round of the French presidential election, exit polls suggest.

Early results give Hollande 28% of the vote, while incumbent conservative Nicolas Sarkozy received 26%.

Far-right candidate Marine Le Pen outperformed pre-election polls, receiving about 20% of the vote, according to the BBC.

The official projections gave Hollande 28.6%, Sarkozy 27%, Le Pen 19%, left wing candidate Jean-Luc Mélenchon 10.8%.

It is the first time since 1958 that the incumbent president failed to win the first round of a re-election campaign.

Speaking at a rally of his supporters following the result, Hollande said the result was a "punishment" for Sarkozy.

"The outgoing president has been disavowed," he said. "I want to thank the voters who have put me in that position which does me honour."

Officials reported a strong turnout in the early voting, with the Interior Ministry reporting 70% of all votes cast before 5pm. Local media said that around 80% participation was expected by the time polls close in large cities.

After Sunday's poll an initial list of 10 candidates will be cut down to two before a 6 May runoff contest.

Centre-right incumbent Sarkozy will face a strong challenge from Socialist nominee Francois Hollande in the second round.

Sarkozy's popularity has fallen in the past few weeks, with voters said to be looking for hard solutions on jobs and economic growth.

Hollande has promised to raise the minimum wage while raising corporation tax and levies on high earners. He has also pledged to hire more teachers and lower the retirement age for some workers to 60.

Sarkozy has pledged to tackle immigration by pulling out of the "passport free" zone unless other countries do more to reduce numbers of illegal migrants entering the EU. He has also campaigned on reducing France's defect and promoting economic growth.

He has been in office since 2007, and if he loses would be the first president since 1981 not to receive a second term from French voters.

Both main candidates have pushed for a strong turnout, the AP reported, in order to dilute the impact of fringe candidates including far-right nationalist Marine Le Pen, daughter of the 2002 second-round contender.

The eventual result could have massive repercussions for Europe, given Sarkozy's strong support with German Chancellor Angela Merkel for budget austerity in the eurozone.

Hollande has said that a new treaty in the eurozone will also have to help states support economic growth through spending.

Speaking after the first round of voting was complete, Le Pen said it was "only the beginning" and told her supporters that "now for us everything is possible".

"Millions of french people this evening have begun to resist, this is only the begging lets go on fighting," she said.

"All these people have been carrying the blue Marine wave that makes the power tremble."

On Twitter French voters used a satirical search term, '#radioLondres', to get around extremely stringent laws prohibiting the publication of exit polls or other surveys before voting ends.

Anyone leaking the results would face a fine of up to $100,000.

The #RadioLondres topic, the name of which references foreign radio broadcasts from World War Two, highlighted pranksters jokingly referring to the price of maple syrup in two countries as code for the results.