Matteo Renzi said Monday that he was quitting as leader of the Democratic Party (PD) after its poor showing in Sunday's general election. "It's obvious that I must leave the helm of the Democratic Party," Renzi said.
The ex-premier said the centre-left group must "turn a new page" after suffering a "clear defeat".
He said a party congress would be held to select a new leader after the new parliament and government has been sworn in.
The PD is set to win under 20% of the vote in the election, which saw the anti-establishment 5-Star Movement (M5S) finishing as the top individual party and the centre right come in as the top coalition.
The PD got over 40% in the 2014 European elections early in Renzi's 2014-2016 premiership.
Renzi said the PD will not team up with "extremists" to form a new executive. "I feel that I'm the guarantor of a moral, political and cultural commitment," Renzi told a news conference, singling out the M5S and the anti-migrant, Euroskeptic, rightwing League, the two big winners of the vote, as the main extremist forces.
"We said no to a government with extremists during the election campaign and we have not changed our minds. We weren't joking".
He listed three main differences with the League leader Salvini and M5S premier candidate Luigi Di Maio - "their anti-European positions, the anti-politics positions and the use of verbal hatred.
"If they say we are mafiosi, corrupt, unfit to stand with our hands soiled with blood, you know what? You can govern without us, out place in in the opposition".
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