Francois Hollande became France's first socialist president in 17 years on Sunday, ousting conservative incumbent Nicolas Sarkozy in what one observer called an "earth-shaking" election.

With 95 per cent of the ballots counted, official results showed Hollande with 51.6 per cent of the vote compared with Sarkozy's 48.4 per cent. More than 80 per cent of eligible voters cast their ballots in the run-off election.

Hollande declared victory shortly after Sarkozy called to wish him good luck as the new leader of France. He told a huge crowd of cheering supporters in Tulle, central France, that he was "proud to have been capable of giving people hope again."

"We will succeed," Hollande said.

Speaking at Place de la Bastille in Paris later Sunday, Hollande promised he will push for change across Europe.

"In all the capitals ... there are people who, thanks to us, are hoping, are looking to us, and want to finish with austerity," he said.

"Too many divisions, too many wounds, too many breakdowns and divides have separated our fellow citizens. This is over now."

Hollande has campaigned on a promise to promote government stimulus programs rather than focus on harsh austerity measures Sarkozy had favoured.

He is the first leftist leader of France since socialist Francois Mitterand left office in 1995.

Sarkozy, who was widely criticized for his handling of the economic crisis and France's unemployment rate, thanked his supporters and said he did his best to win a second term.

"I take responsibility ... for the defeat," he said in his concession speech in Paris.

Over the past few weeks, analysts have been predicting Sarkozy's demise amid widespread voter anger over the economic downturn and forced austerity measures.

During Sarkozy's five-year term, France's unemployment rate surged to almost 10 per cent, the economy stalled and the country found itself in the grips of the eurozone crisis.

The global financial situation forced controversial austerity measures in many European countries, including France, prompting disappointment from citizens who feel they are worse off now than they were before.

Under Sarkozy, France pledged to rein in its spending while the rest of 17 countries that use the euro embark on a strict period of belt-tightening. In France, that has included programs designed to reduce government employment.

Hollande has said that his first task as president will be to call for a renegotiation of the European budget-trimming treaty, spearheaded by Sarkozy and German Chancellor Angela Merkel. Hollande wants to see more government-funded stimulus programs instead of drastic cuts across the board.

Hollande has also pledged to tax the very rich at 75 per cent of their income, and allow some people to retire at age 60 instead of 62. Retiring at 62 became law under Sarkozy's rule, and sparked labour strikes and street protests.

"The voters made a big choice…and they chose to move away from some of the more extreme austerity (measures) and move towards a more humane policy to create jobs and grow the economy," American political commentator Brent Budowsky, who has been closely watching the French vote, told CTV News Channel on Sunday.

"It's one of these earth-shaking elections that I think will have enormous implications," he said.

Budowsky said Sarkozy erred by shifting too far to the right in response to Europe's fiscal problems and trying to push "extreme" austerity measures.

Sarkozy was also disliked by many voters for his "flashy" personality. Critics have often faulted him for his brash style and alleged chumminess with the rich.

Had Sarkozy been as gracious during the election campaign as he was in his concession speech Sunday, "it would definitely have been a closer election and he might have had an outside chance of pulling it off," Budowsky said.

Instead, Sarkozy's "demeanor was much too antagonistic and too aggressive," he said.

Hollande has been campaigning on a more balanced approach to job creation and economic growth. His presidency and France's shift to the left will have "profound implications" for the rest of Europe and even the U.S., Budowsky said.

As for Merkel, she will likely have to shift to the centre of the political spectrum in order to work closely with Hollande, Budowsky added.

Merkel and U.S. President Barack Obama were among the world leaders who called Hollande Sunday to congratulate him on his victory. He will take office no later than May 16.

With files from The Associated Press