French Prime Minister Manuel Valls on Monday threw his hat in the ring to succeed Francois Hollande in next year's presidential election, AFP reports.
"I am a candidate for the presidency of the Republic," Valls said, announcing he would step down as premier on Tuesday to campaign for the Socialist nomination in a primary in January.
The 54-year-old Spanish-born politician announced his candidacy in a speech from his political base in the tough Paris suburb of Evry.
"I have a responsibility to unite," he said, appealing to the fractured left to rally behind him.
He warned of the risk of far-right leader Marine Le Pen pulling off a repeat of France's 2002 electoral upset when her father Jean-Marie Le Pen edged the Socialist candidate for a place in the presidential runoff.
Le Pen, he said, would "take us out of Europe" and her policies would "ruin the working class".
Polls show the younger Le Pen winning or being placed second in the opening round of the election on April 23 but later being defeated by the conservative Francois Fillon.
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.
Valls said his candidacy was "a revolt against the idea that the left has been disqualified".
His 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 after a troubled four years.
Health Minister Marisol Touraine and Interior Minister Bernard Cazeneuve are among those tipped to succeed Valls as premier for the six months until the legislative elections that follow the presidential vote.