Thousands of protesters marched through the Polish capital on Saturday demanding higher public sector pay, piling pressure on the Law and Justice (PiS) government ahead of a municipal election in October.

PiS, an economically left-leaning party with a conservative social agenda, swept into power in 2015 on a promise of massive welfare spending, more traditional values in public life and a boost in support for less-privileged Poles.

But trade unions are demanding more state money, particularly for salaries in the public sector, including education and healthcare, as the economy powers ahead at a rate that's among the highest in Europe.

Poland's gross domestic product has expanded by four to five percent on an annual basis each quarter since the beginning of last year, propelled largely by domestic consumption.

But economists warn that Poland may begin to suffer from weaker economic sentiment prevailing across Europe, wage pressures and labour shortages in the coming years. And the PiS has stopped making promises on large-scale welfare spending.

"We can't live pay cheque to pay cheque ... Our families are starving," Jan Guz, head of the OPZZ trade union told protesters. OPZZ, which organised the march, has some 500,000 members across the country.

Another trade union, NSZZ Solidarnosc, a bedrock supporter of the PiS in past elections with nearly 700,000 members, last month called for a 12 percent pay rise to make up for inflation since public sector wages were last indexed in 2010.

Public support for PiS has held above 30 percent since the 2015 parliamentary election. Aside from the municipal election in four weeks, Poles will vote in a European Parliament ballot next May and in a parliamentary election in late 2019.

Warsaw authorities said some 26,000 people took part in the protest.

Since coming to power, PiS has introduced cash stipends for families, raised salaries for young teachers, policemen and doctors, increased the minimum wage and lowered the retirement age in the country of 38 million.