Thousands of workers belonging to Poland's historic Solidarity trade union staged a massive protest in Warsaw Friday, demanding wage hikes and pension privileges for public sector workers.

The massive protest snarled traffic in the capital, with organisers claiming a 30,000-strong turn-out. Police pegged the demonstration at 18,000.

Bearing banners reading "We're workers, not slaves" amid a heavy police presence, the protesters marched through the city centre setting-off fire crackers, smoke bombs and jeering "thieves," before stopping in front of government buildings where they burned an effigy of liberal Polish Prime Minister Donald Tusk.

Unionists argue salaries for some public sector workers, particularly medical professionals, teachers and miners, are too low.

Solidarity was also protesting government plans to reform retirement benefits in a way that would deprive certain public sector workers of the right to early retirement or allow them only limited pension benefits should they retire early.

"Workers are the greatest treasure of the Polish economy and there must be investment in human capital," union head Janusz Sniadek said, rallying demonstrators who braved driving rain.

"Jobs must be secure, well managed, decently paid and have honest retirement benefits which do not require elderly people to turn to social assistance," he said.

Protesters forwarded a petition to Prime Minister Tusk who was not in Warsaw for the demonstration, organised on the eve of Solidarity's 28th anniversary.

The historic August 31, 1980 Gdansk accord between striking shipyard workers under yard electrician Lech Walesa and Poland's communist party gave birth to the union, the first and only independent trade union in the entire Soviet bloc.

It was a key player in the 1989 Round Table agreements with the communist regime which brought a peaceful, negotiated end to communism in Poland in 1989.