“Today I have decided to quit the Socialist Party,” Hamon told a crowd he claimed was 11,000-strong. “I am leaving the party but I am not giving up on the socialist ideal.
“The Socialist Party has perhaps had its day … My belief is that it is time to turn the page.”
But he said the move was a personal choice and he was not asking other Socialist Party members to follow his example.
Hamon, who won just 6.3 percent in the first round of France’s presidential election, founded his 1 July Movement earlier this year and said he hoped it will become one of the “beams” of the “house of the left”.
The Socialists lost voters to President Emmanuel Macron’s Republic on the Move (LREM) and Jean-Luc Mélenchon’s France Unbowed (LFI) and “the French people have begun to abandon the Socialist vote”, he said.
Former prime minister Manuel Valls, who lost to Hamon in the Socialist primary and backed Macron in the presidential poll, left the Socialists last week after narrowly winning reelection to parliament.
Hamon accused Mélenchon for “populism”, accusing him of taking the risk of building “intellectual bridges” to the far right in his efforts to win a working-class electoral base.
But he was tougher on Macron, who he said embodies a “social minority” that has chosen to “rule for itself”.
In an interview published Sunday but given before Hamon’s announcement, Mélenchon said that the “old left” – not only the Socialists but also the Communist Party and the Greens – was in the process of decomposition after suffering a “terrible electoral defeat”.
After winning seven million votes in the presidential election and 17 seats in parliament, his movement will present a “global and frontal” opposition to Macron, he told the Journal du Dimanche, promising to “inform and mobilise” and, if needs be, organise protests movements.