Mr Renzi has refused to form a coalition with any of the nationalist parties that proved so popular with the Italian public and has announced he will quit his leadership once a government is formed.
Alessandro Di Battista of the anti-establishment Five Star Movement (M5S), which has emerged as Italy’s largest party, said: “Renzi? I have never listened to such a bizarre speech, he is really confused.
“In order not to actually resign, he is willing to shatter what remains of the Democratic Party.
“Our line does not change. Democratic Party? It is already without Renzi”.
Mr Renzi, 43, who led the country from 2014 to 2016, announced he will leave the leadership of the party after “the establishment of the government”.
He said: “It’s obvious that I must leave the helm of the Democratic Party.
“We will be in the opposition: the PD will never act as a front for government made up of anti-systemic forces.
“There are three things that distinguish the PD from the anti-establishment 5-Star Movement and League: their anti-Europeanism, their anti-politics and the verbal hatred they have directed at Democratic Party members.”
According to preliminary results, the PD looked set to come third with about 19 percent of the vote — less than half their peak at the European elections in 2014 — while the 5Star Movement emerged as the single largest party with more than 32 percent.
Mr Renzi’s popularity, and support for his party, suffered the consequences of Italy’s slow economic recovery and persistently high unemployment.
Once nicknamed ‘the bulldozer’ for his reformist zeal, the former mayor of Florence bet his political career on a referendum to change the constitution in 2016 — and lost.
After Sunday’s inconclusive vote, no single party or coalition looks likely to be able to collect enough votes to govern alone, which could result in a hung parliament.
Additional reporting by Maria Ortega