Nicola Zingaretti, the president of Italy’s Lazio region, has become the new leader of the centre-left Democratic party after capturing a projected 67% of the vote in an election on Sunday.

More than 1.6 million party supporters cast their ballots across the country in a turnout that was higher than expected, giving the party hope that it can re-establish itself as a credible force against Italy’s populist government.

“Long live Italian democracy, which gives us lessons whenever it can,” said Zingaretti. “I am happy for Italy.”

The vote was seen as a litmus test for the strength of Democratic party and comes a year after its administration was ousted in an embarrassing defeat in general elections that led, eventually, to the formation of a coalition government between the far-right League and anti-establishment Five Star Movement (M5S). The trouncing deepened rifts within the party which, until the national elections, was led by former prime minister Matteo Renzi, whose confrontational style also lost supporters.

Renzi congratulated Zingaretti on Sunday, saying he won “a wonderful and clear-cut victory”. “Now this friendly fire must stop,” he added. “Our political opponents are not at home but in the government.”

Zingaretti, whose brother Luca plays the title role in the TV series Inspector Montalbano, has led Lazio since 2013. He is regarded as a moderate leftwinger capable of uniting the party. The other two main candidates were Maurizio Martina, the party’s former caretaker leader, and Roberto Giachetti.

Nicola Zingaretti casts his ballot in Rome. Photograph: Angelo Carconi/AP

The Democratic party has been accused of losing touch with voters, something that paved the way for Matteo Salvini, the deputy prime minister and leader of the League, to thrive. The League leads in national polls, while support for M5S has slipped.

Supporters of the Democratic party argue that it must now unite, particularly as it is fighting European elections in May, and convince people that it is a strong alternative.

“The important thing today is that there is big participation,” said Mauro Valorani, a professor in Rome who backed Zingaretti. “We need to send a strong message that we don’t like what is going on in Italy and elsewhere in the EU. There is a far-right movement building across Europe that is much better organised than it used to be. Europe is a stronghold of democracy and we need to take care of this.”

Giuseppe DeFinis, who also backed Zingaretti, said he voted “because the danger to Italy is Salvini”. He said: “He represents an obscure part of fascism, hatred towards diversity.”

Italian deputy prime minister Matteo Salvini. Photograph: Tony Gentile/Reuters

Franco Pina, who voted for Martina, said the situation was “dangerous and xenophobic”. He said: “The Democratic party leadership has made mistakes in the past, but now we must unite.”

The three leadership candidates were among the estimated 200,000 people who converged in Milan on Saturday for a demonstration against racism.

“It’s important to underline this demonstration,” said Mattia Diletti, a politics professor at Rome’s Sapienza University. “Not because of the Democratic party, but because it shows that society is pushing back against Salvini’s thoughts and hegemony.”