French Prime Minister Manuel Valls on Monday threw his hat in the ring to succeed Francois Hollande in next year's presidential election.

"I am a candidate for the presidency of the Republic," Valls said, announcing he would step down as prime minister on Tuesday to campaign for the Socialist nomination in a primary in January.

The 54-year-old Spanish-born premier announced his candidacy in a speech from his political base in the tough Paris suburb of Evry, in which he appealed to the fractured left to unite behind him.

"I have a responsibility today -- to unite," Valls, seen as a divisive figure, said.

He warned of the risk of far-right leader Marine Le Pen shutting the left out of the decisive second round of the election in a repeat of France's 2002 electoral earthquake when her father Jean-Marie Le Pen was beaten in the runoff.

Valls' entry into the race had been expected after Hollande announced last week he would not seek a second term, bowing to pressure to step aside, including from his prime minister.

Health Minister Marisol Touraine and Interior Minister Bernard Cazeneuve are among those tipped to succeed him as premier for the six months until legislative elections that follow the presidential vote.

The combative Valls meanwhile will go up against at least seven other candidates for the nomination of the mainstream left in a two-round primary on January 22 and 29.

The first round of the presidential election in France is due to be held on April 23 with a run-off on May 7.

Polls show Marine Le Pen could come first or second in the first round, but would likely lose to the conservative candidate Francois Fillon in the second round.

But with Sunday's Italian referendum defeat for Prime Minister Matteo Renzi just the latest rout of mainstream political figures at the ballot box, no-one is ruling out a Le Pen victory.

The divided French left currently appears ill-equipped for such a fight.

Valls is a polarising figure on the left, accused of being a closet conservative for using decrees to force labour reforms through parliament and endorsing controversial bans last summer on the Islamic "burkini" swimsuit.

Martine Aubry, a veteran Socialist and the mayor of the northern city of Lille, said Monday she did not believe Valls could unite the Socialists.

Polls currently show Valls winning the primary but trailing behind Le Pen and Fillon in the first round of the presidential election.

Valls would face a fight for votes with two candidates who are not standing in the nominating contest -- business-friendly former economy minister Emmanuel Macron and the fiery hard-left Jean-Luc Melenchon.

Valls sought the Socialist nomination five years ago as a rank outsider, winning only 5.6 percent on a platform seen as too economically liberal.

He became a spokesman for Hollande's campaign and when the Socialist won the presidency he rewarded Valls with the post of interior minister.

In 2014, Valls was promoted to prime minister with a mandate to rein in a group of unruly ministers that were undermining Hollande's authority.

Within months he had a rival for the title of reformer-in-chief, in the telegenic banker-turned-economy minister Macron.

Hollande's protege walked out on the government in August to further his own presidential ambitions -- infuriating Valls who accused him of "destroying the left".

Valls said Friday he was determined to "defend the legacy" of Hollande, despite opinion polls showing that his former boss is the least popular French president for nearly 60 years.

But in a newspaper interview last month, the normally loyal Valls had himself turned on Hollande, saying explosive revelations contained in a book of interviews between the president and two journalists had "plunged the left into total disarray".

Valls said he wanted to "dispel the notion that defeat is inevitable" for the Socialists.