ATHENS — Greece's conservative prime minister Kyriakos Mitsotakis has won national elections, hailing his party's big victory as a "political earthquake".

His center-right New Democracy party were heading for almost 41% of the vote, five seats short of a majority.

His center-left rival Alexis Tsipras congratulated him, with his Syriza party set for a poor result of 20%.

Mitsotakis said the result showed that Greeks had given his party a mandate for a four-year government.

"The people wanted the choice of a Greece run by a majority government and by New Democracy without the help of others," he said in a victory speech.

Hours earlier party supporters in Athens cheered as an exit poll indicated the unexpected scale of New Democracy's victory. As results emerged, it was clear that pre-election polls had underestimated the 20-point margin between the two main parties.

With almost all the results counted, Mitsotakis's party was set to win 146 seats, five seats short of the 151 required for a majority. An interior ministry vote map showed all but one of Greece's electoral districts colored in New Democracy blue.

The prime minister's remarks were taken as an indication that he would not look to share power with another party but go for a second election in late June, when the winning party picks up bonus seats.

Greek President Katerina Sakellaropoulou will offer him the chance of forming a coalition, but it is already clear he will refuse.

The result was an immense setback for Syriza leader Alexis Tsipras, who described his party's performance as "extremely negative". He came to power in 2015 campaigning against the austerity of international bailouts but ultimately agreed to creditors' demands.

The center-right has governed Greece for the past four years, and can boast that the country's growth last year was close to 6%.

Mitsotakis's pitch to the nation was that only he could be trusted to steer the Greek economy forward and consolidate recent growth. Greeks appear to have responded positively — more than expected.

Giorgos Adamopoulos, 47, voted for New Democracy a few hundred metres from the Acropolis in Athens.

Greece deserved a better form of politics, he told the BBC, but he backed Mitsotakis because he was impressed with his record after four years as prime minister.

Four years ago winning 41% of the vote would have been enough to secure a majority in Greece's 300-seat parliament.

Now it requires more than 45%, because the winning party is no longer entitled to a 50-seat bonus in the first round, making a re-run more likely.

Mitsotakis will have his eye on the extra seats he would be entitled to if he won the second election. An outright majority would give him four years in power with a cabinet of his choice.

If he were to seek coalition talks, then Syriza's socialist rival Pasok would be a potential partner, as one of the election's big winners with 11.5% of the vote.

But that would prove tricky as Pasok leader Nikos Androulakis was the target of a wiretap scandal last year.

It led to the resignations of a nephew of Mr Mitsotakis, who was working as the prime minister's chief of staff, and also of the head of Greek intelligence.

Androulakis believes the prime minister was aware he was one of the dozens of people targeted with illegal spyware.

Mr Mitsotakis comes from one of Greece's most powerful political dynasties.

His father Konstantinos Mitsotakis was himself prime minister in the early 1990s; his sister Dora Bakoyannis was foreign minister and her son Kostas Bakoyannis is the current mayor of Athens.

In the end a rail tragedy in February that overshadowed the election campaign played no obvious role in the result.

57 people died in the disaster, many of them students. Opposition parties highlighted the tragedy as a symptom of a dysfunctional state pared down to the bone after years of economic crisis and under-investment.

Greeks have the right to vote from the age of 17, and an initial analysis of voting by Greek TV suggested that 31.5% of voters aged 17-24 backed ND, almost three points higher than Syriza.

First-time voters Chrysanthi and Vaggelis, both 18, voted for Syriza because their generation wanted "something new, something different".

Other than Pasok, the communist KKE party also increased their share of the vote.

But another casualty was former Syriza finance minister Yannis Varoufakis, whose MeRA25 party failed to qualify for parliament. — BBCa