Trade union members had arrived in buses from throughout the country and before the rally formally began demonstrators were chanting slogans against the government of Prime Minister Zoran Milanovic. Police put the number of protesters at 13,000 and said that only one person was arrested for disorderly conduct.

The unions demanded a change of the economic policy, an end to the policy of austerity, job creation, preservation of the welfare state, taxation of banks and the finance industry, and penalties for failure to pay wages.

Union leaders warned the government that it was its last chance to change its policy or it should start considering an early election after July 1, when Croatia is set to join the European Union. They also criticised the strongest opposition party, the Croatian Democratic Union (HDZ), saying that the unions would be looking closely what the HDZ was doing.

Union leader Vilim Ribic said that the present government had betrayed everyone except bankers, the International Monetary Fund and big business, and stressed that workers did not need a government like this. He said that the HDZ was not proposing better solutions either and that therefore Croatia needed a third political option.

"This country needs a third political force that will be able to turn on the light at the end of the tunnel," Ribic said.

Another union leader, Kresimir Sever, warned that the country was sinking into resignation and hopelessness as a result of the government's policies. "We were not brought here by the opposition, but by misery, poverty and prospects of a bleak future into which Prime Minister Zoran Milanovic and his government are pushing us," he said.

Prime Minister Milanovic was the target of severe criticism from speakers and was booed by the crowd. Union leaders accused him of betraying the working class, the welfare state, his voters and the idea of social democracy, and of serving the interests of big business and banks.

The unions called for reinstatement of a maximum income tax rate of 45%, which had been reduced to 40% by the previous HDZ-led government.

The rally lasted for about two hours, after which the crowd dispersed. Police did not report any incidents.

Meanwhile, thousands of residents of Zagreb flocked to the city's Maksimir Park for a traditional May Day celebration organised by the city authorities, with a programme of music, entertainment and free servings of cooked beans.

They were joined by politicians, and in greater numbers this year because of forthcoming local elections. Among them was Mayor Milan Bandic, who is vying for a fifth term in office, and his rival Rajko Ostojic of the Social Democratic Party (SDP). Vladimir Ferdelji, the candidate of the coalition of the Social Liberal Party, Peasant Party, Green List and Democratic Party of Pensioners, was also expected to make an appearance.

Also there was President Ivo Josipovic, who said that today's protests were legitimate because "many people have reason for dissatisfaction."

Josipovic said that this year too International Workers' Day in Croatia was being marked against a background of an economic crisis when "many people don't have work and some of the workers don't get their wages."

"The most important task is to create jobs, get the economy going and encourage investment," Josipovic told the press on arrival in Maksimir where he was greeted with applause from the crowd.

Commenting on the protest rally in Zagreb, Josipovic said he was confident that its message would be constructive "because we know what the shortcomings are and what we are to do." When asked if it would have any effect, he said that "the effect comes from work, investment and the government".

May Day celebrations were held throughout Croatia on Wednesday.