WARSAW, Poland — Pope Benedict XVI celebrated Mass for an estimated 270,000 people Friday in a rain-soaked Warsaw square where his predecessor, John Paul II, inspired Poland's Solidarity movement against communist rule in a historic 1979 visit.

In his sermon, Benedict challenged moral relativism, or the view that there are no absolute values, and defended the church's unchanging traditional beliefs.

In remarks read in Polish by an aide, Benedict warned the faithful against those "seeking to falsify the word of Christ and to remove from the Gospel those truths which in their view are too uncomfortable for modern man."

"They try to give the impression that everything is relative: even the truths of faith would depend on the historical situation and human evaluation," he said, in remarks that echoed his homily at John Paul II's funeral last year. "Yet the church cannot silence the spirit of truth."

The choice of site — called Victory Square in 1979 and today Pilsudski Square — harkened back to John Paul II's challenge to "renew the face of the Earth, of this land" during his triumphant first trip to his native land after being elected pope.

That visit challenged the atheist Communist authorities and is credited by Solidarity founder Lech Walesa with inspiring trade union resistance to Soviet-backed rule, which collapsed in 1989-90.

Spectators stood resolutely Friday in ponchos and under umbrellas, filling vast Pilsudski Square before an 82-foot metal cross on an elevated platform.