Buses of Polish security forces are moving into Krakow as thousands of young people gather ahead of the arrival of Pope Francis for World Youth Day, a major gathering of Catholics from around the world.

Those gathered expressed shock at the killing in France on Tuesday of an 86-year-old priest whose throat was cut by two hostage-takers in a church.

Archbishop Georges Pontier, president of the French bishops conference, said in Krakow that the attack "shocked us, it touched us, and revolted us."

He also called on Christians not to be discouraged by such inhumane acts.

After a string of attacks in Europe, Polish officials are trying to give reassurances that the nation's security forces are prepared for the massive event, which the pope will attend from Wednesday to Sunday.