Greek Prime Minister Kyriakos Mitsotakis on Friday warned voters to avoid "experimenting" with protest parties in upcoming European Parliament elections, warning that the country's "stability" was at stake. 

"We ask citizens to say no to irrelevant experimentation, and a big yes to stability," Mitsotakis said on the opening day of a three-day congress of the ruling New Democracy party. 

"More than ever, we need unity in society and continuity in the economy," he said. 

One of Europe's strongest conservative parties, New Democracy steadily leads opinion surveys after a comfortable second-term victory for pro-business Mitsotakis in national elections last June. 

A Pulse poll for Skai TV this week found New Democracy was in the lead ahead of the European Parliament vote in June with 28.5 per cent, with main opposition party Syriza a distant second at 13 per cent.  

The lead persists despite a wiretapping scandal and Greece's worst train disaster a year ago that left 57 dead. 

On Monday, there was shock when a woman was fatally stabbed by her allegedly abusive former partner outside an Athens police station, where she had gone to seek help. 

"When a young woman is murdered outside a police station where she has sought refuge after being threatened... we say we must do more," Mitsotakis, who has campaigned on law and order, said Friday. 

"(Police) ought to immediately intervene when a crime occurs in front of them."  

"Incidents such as these tarnish whatever positive we have achieved," he said. 

European Commission chief Ursula von der Leyen is scheduled to address the congress on Sunday. 

The conservative European People's Party (EPP) chairman Manfred Weber will speak on Saturday. 

The EPP last month endorsed von der Leyen's bid for a second term.