Foreign leaders have paid tribute to Poland's Solidarity movement at events to mark the founding of the first free trade union in the former Soviet bloc.
The event is being marked by three days of celebrations
European Commission President Jose Manuel Barroso said the movement had launched an irreversible process towards freedom in eastern Europe.
He was speaking at a conference in Gdansk, the birthplace of Solidarity.
Thousands of people then attended a Mass by the shipyard where Lech Walesa and fellow workers founded Solidarity.
In the shipyard itself, about 200 former and present day Solidarity activists opposed to the official celebrations protested against job losses in recent years.
Solidarity (Solidarnosc), officially recognised on 31 August 1980, is credited with playing a key role in the collapse of communism.
Within months of its establishment, Solidarity had become a national political movement claiming 10 million members.
Nine turbulent years later, Solidarity leaders negotiated the end of communism and a few months later, the Berlin Wall fell.
Poland is holding three days of celebrations for Solidarity's 25th anniversary, with the open-air mass in Gdansk one of the highlights.
Heads of state joined Solidarity members and former activists for the ceremony led by the Archbishop of Krakow, Stanislaw Dziwisz.
Among the speakers was Mr Walesa, the unemployed electrician who led the city strike that led to the formation of Solidarity. He went on to become the country's president.
Mr Walesa challenged the European Union to work for the unity of Europe, torn by recent rows over budget and future shape.
"I don't want the European Union to be something where France is pulling its way, Germany its way and Poland and others another way," he said.
"This is not something a worker from Gdansk fought for."
The Mass marked the role played by the Roman Catholic Church in toppling communism, particularly that of Poland's Karol Wojtyla, then newly-elected as Pope John Paul II.
In a letter read out in Polish by Archbishop Dziwisz, Pope Benedict XVI praised Solidarity as a "breath of a new spirit" that changed Europe.
"I know how much it warmed the heart of my great predecessor, God's servant John Paul II, that this act of historic justice happened and that Europe was able to breathe with two lungs - an eastern one and a western one," the letter said.