By Christian Lowe and Rob Strybel
WARSAW (Reuters) - Tens of thousands of protesters marched through the Polish capital on Saturday, the start of an opposition drive to capitalize on the spluttering economy and try to loosen Prime Minister Donald Tusk's grip on power.
Poland's economy, the biggest in central Europe, has grown robustly even while its neighbors slipped into recession, handing Tusk high levels of support and leaving his opponents struggling to win credibility with voters.
Economic growth is expected to slow to just above 2 percent next year. That is healthy by the standards of most European countries but a jarring deceleration for Poles used to two decades of uninterrupted growth.
Jaroslaw Kaczynski, leader of the opposition Law and Justice Party, said ordinary Poles were no longer prepared to give Tusk's government, in its second term, the benefit of the doubt.
"These huge crowds mean strength. They mean that Poland has awakened. More and more Poles will be awakening. The cup of evil has overflowed. We Poles, we Polish patriots say 'no'," Kaczynski told protesters in Warsaw's Castle Square.
"Tusk will probably stand up and say he is the leader for difficult times. Do you want such a leader?" he said. "Honest businessmen and the working masses do not want that."
People close to the government say the downturn could prove the biggest political test yet for Tusk. He has established a reputation for steady and competent stewardship in a part of Europe more familiar with mercurial leaders.
Nevertheless, analysts and diplomats say the opposition is unable to exploit the slowdown because, while it has a strong base among the most devout of Poland's Catholics, it is too hard-line to win over voters in the middle ground.
It is against abortion, gay marriage and in vitro fertilization. That stance is in line with the teachings of the Catholic church, which is still powerful in Poland. But surveys show it does not resonate with young, urban middle classes.
Saturday's demonstration was the biggest in several months. Protesters, some of them clutching rosaries and portraits of the Virgin Mary, knelt on the tarmac in Warsaw's Three Crosses Square as a priest said mass before the march.
Banners demanded more generous terms for pensioners and a broadcast license for a television station run by a conservative Catholic cleric who opposes the government.
One grey-bearded protester, who gave his name as Wojciech, said the Communist cabal which used to rule Poland before the fall of the Berlin Wall had been replaced by another elite, this one preaching free market economics.
"Unemployment is so high," he said, holding up one end of a Law and Justice banner. "Under Communism you could get hit by a truncheon, and now the truncheon is the economy."
Tusk's government does not face re-election for another three years. The latest opinion polls show the prime minister's Civic Platform party still has a significant lead over the opposition, though the gap has narrowed. (Additional reporting by Patrycja Sikora; Editing by Robin Pomeroy)