More than 1,000 people gathered on Saturday in the Croatian capital at a left-wing opposition protest less than a month before a general election, accusing the ruling conservatives of corruption.

The protests called "It's enough!" were held also in four other major Croatian cities.

The parliamentary election in the European Union country will be held on April 17.

The demonstrations were organised by the left-green Mozemo ("We Can" in Croatian) party and a coalition of six left-wing parties led by the main opposition Social Democrats (SDP).

An anti-government rally in Zagreb last month attracted several thousand people.

On Saturday, the protesters marched through downtown Zagreb carrying red banners that read "Enough" and waved Croatian flags.

They chanted slogans against Prime Minister Andrej Plenkovic and the ruling HDZ party.

"Enough of crime! Enough of HDZ! Enough of corruption!", SDP leader Pedja Grbin told the crowd, pledging to "finish the job at the polling stations".

Snjezana Kirinic, a 46-year-old engineer, told AFP that she was protesting to show that "we have had enough of the corruption and tyranny of Andrej Plenkovic, who does not allow the functioning of independent institutions".

Croatia's political scene was rocked earlier this month by the announcement of President Zoran Milanovic that he wanted to stand as an SDP candidate in the April vote.

The country's top court blocked his bid ruling that he would have to resign before making any attempt to become prime minister -- a post he already held from 2011-16.

But Milanovic has vowed to replace Plenkovic as the country's prime minister.

If the SDP and its allies win enough seats in the election, Milanovic could then resign and be picked to head a ruling coalition -- which would be an unprecedented move.

Political analysts say that the president, who tops opinion polls, has significantly boosted the chances of the SDP and other left-wing parties.

Reform of the judiciary and fighting corruption were among key criteria Croatia had to meet to eventually join the EU in 2013.