Tens of thousands of Polish trade unionists kicked off a march through the capital Saturday in the finale of a four-day protest against the unpopular and increasingly fragile centre-right government.

The protest reflects widespread public gloom over this year's sharp economic downturn in Poland, which has felt the drag of the Eurozone crisis.

Disillusionment has plunged the coalition government's popularity to its lowest level since Prime Minister Donald Tusk took office in 2007.

Marek Lewandowski, spokesman for the Polish trade union federation Solidarity, told AFP: "We want the departure of Donald Tusk. This is the only way to change social policy in Poland."

Since dawn, hundreds of buses with protesters have descended on Warsaw.

Lewandowski said the right-of-centre Solidarity was expecting 100,000 of its members to join the protest, with two other unions also expected on the streets Saturday.

They have the support of a majority of Poles, according to an opinion poll carried out by MillwardBrown for the Fakty news programme on Tuesday. Some 59 percent of respondents said they were for the demonstrations, with 31 percent against and the remainder unsure.

Three of Tusk's MPs have also defected in recent weeks, raising the spectre of a minority government that could spell early elections ahead of 2015.

While the unionists have a long litany of grievances, their anger is centred on recent labour and pension changes.

"We want pensions at the age of 65 as before and not at the age of 67 as Tusk's reform would have it. We want better social policy and guarantees for employees," Lewandowski said.

The protesters began their action Wednesday, gathering in front of the main ministries before marching towards parliament to the sound of drums, whistles and sirens.

The union is also calling for a higher minimum wage and improved job security — particularly for young people — in the EU member country.

"I want an end to temporary contracts," said Zdzislaw Urabanek, a chemical plant worker from the eastern town of Pulawy.

"Young people are only getting contracts for one, two, three months," the 60-year-old Solidarity member told AFP.

Unions also accuse Tusk of ignoring their demands and refusing to engage in dialogue.

Lawmaker Jacek Zalek, who quit Tusk's centrist Civic Platform (PO) on Thursday, told reporters the "protests [are] a sign that we were unable to rise to the challenges that were put before us by Poles."

But Solidarity's cozy alliance with the populist right-wing opposition led by former prime minister Jaroslaw Kaczynski — the twin of late President Lech Kaczyński — gives Saturday's protest a razor-sharp political edge.

Recent opinion polls have consistently shown his Law and Justice (PiS) Party running ahead of the PO, whose majority is now down to a fragile 232 seats in the 460-member lower house of parliament.

But Tusk argued Friday that his success in pushing through a revised 2013 budget — widening the deficit by an extra 3.8 billion Euros ($5 billion) — bodes well for the future.

"Today's vote proves that the government's majority is stable," he said after Friday's vote, adding that concerns over the exodus of MPs are "unfounded."

Lawmakers approved the amendment — with 235 votes for, 73 against, and three abstentions — despite a walkout by Kaczynski's entire conservative opposition.

Analysts said the vote suggested Tusk could still muster enough legislative clout to pass the 2014 budget and thus avoid snap elections that the PiS could win.

They also say the party would be unlikely to govern alone and has no ready ally willing to join forces with it.

A central European powerhouse of 38 million people, Poland is the only EU member to have maintained growth each year for two decades.

However, the economy slowed to just 0.1 percent growth in the first quarter of this year as Poland's main trade partner, the Eurozone, struggled with recession.

Growth picked up to 0.4 percent in the second quarter, but by then a 5.6 billion Euro revenue shortfall had emerged.

Tusk said last week he expects the economy to grow by at least 1.5 percent this year, down from the government's original estimate of 2.2 percent.