Croatia's top court rejected Monday a bid by President Zoran Milanovic to stand in the country's coming elections to become prime minister, ruling that he would have to resign beforehand.

Milanovic announced Friday that he wanted to run as an "independent" candidate for the left-wing Social Democratic Party (SDP), the main opposition group, in the April 17 parliamentary elections.

His aim is to oust Prime Minister Andrej Plenkovic, head of the conservative HDZ party and Milanovic's longtime rival, who secured the post in 2016 elections and was re-elected in 2020.

But Milanovic, who already served as premier from 2011 to 2016, said he would not quit as president of the NATO and the European Union country for the time being, a move denounced by the governing conservative party and many legal experts.

The 57-year-old said he would resign as president only after what he called a "certain electoral victory" that would see him become prime minister, who is proposed by the parties that win a parliamentary majority.

The country's constitutional court on Monday said Milanovic's actions were unconstitutional.

The candidacy of a president "while holding the office... and his own highlighting of himself as a candidate for Croatia's prime minister... is incompatible with his constitutional position", the court said in a statement.

If he wants to run he "must resign immediately", it said, ordering Milanovic and the SDP to "immediately cease activities that are contrary to the constitution".

The court also reiterated its power to annul any part of the election process that is deemed unconstitutional.

Milanovic's presidential term expires next February.

Milanovic, known for making inflammatory and populist statements, has said he is promising "a determined and clean government", accusing Plenkovic's administration of corruption.

Thousands of people turned out for an anti-government rally in Zagreb last month, calling for immediate elections and accusing government officials o corruption.