Croatian trade unions handed parliament a petition Thursday calling for a nationwide referendum in a challenge to plans to raise the retirement age from 65 to 67.

The European Union member passed the measure to raise the retirement age starting in 2033 and to cut pensions for those who retire early.

Conservative Prime Minister Andrej Plenkovic's government said the move was necessary to make the pension system sustainable as the country's population is ageing.

But trade unions slammed the changes as "unreasonable", branding them a "consequence of political elitism".

They argued that Croatian living standards and life expectancy were lower than the EU average.

"We are an ill, overworked and poor country in which people are forced to work until exhaustion," said union leader Kresimir Sever.

Unions said they gathered more than 740,000 signatures calling for the referendum during a two-week campaign in April and May.

That is double the number needed to call a referendum under Croatian law.

Unions stacked up 65 boxes containing sheets with signatures outside parliament where they handed them over to parliamentary speaker Gordan Jandrokovic on Thursday.

Once the signatures are verified the parliament may call the referendum or ask the Constitutional Court to rule on whether the issue can be put to such a vote.

"We expect the referendum over the issue to be called soon," union representative Mirela Boric told reporters.

"There is no more space for negotiations" with the government over it, she added.

Unions want the referendum to be called in a few months.