Croatia's Constitutional Court on Monday barred President Zoran Milanovic from running for prime minister in elections next month while still in office, ruling he would have to resign first.

Milanovic, a populist whose presidential term expires in February 2025, served in a more powerful position of prime minister from 2011 to 2016.

Milanovic on Friday set April 17 for the parliamentary vote after dissolving parliament and said he would run for prime minister and resign only "after winning the polls".

The court said the president's role ruled out running as a political candidate.

"The President of the Republic of Croatia is a non-partisan individual and he cannot take part in political activities of any party," Constitutional Court president Miroslav Šeparovic said.

If Milanovic runs for parliament or gives public statements as a future candidate for prime minister, he must immediately file his resignation to the Constitutional Court, Šeparovic said.

"We have found that he violated the constitution by his acts and statements from Friday to today, saying that he will run in the election and giving political statements," Šeparovic said.

Milanovic dismissed the court' ruling as a "constitutional coup d'etat" and reiterated that he would not resign in order to run.

Milanovic's Social Democratic Party (SDP) endorsed him as its candidate for premier when he announced his bid over the weekend. On Monday, its leader Pedja Grbin said the party would comply with the court's ruling but made clear it still backed the president.

"Starting today I am not allowed to say that Milanovic will get a mandate to compile the next Croatian government, that he will be the Croatian next premier at least until the publication of election results," Grbin told a news conference.

"And then we shall shout that."

Šeparovic said the court will monitor the election process and sanction those who violate the constitution, calling on Milanovic and the SDP to halt all anti-constitutional acts. REUTERS