A group of Roman Catholic activists, headed by a U.S. priest who supports ordination for women, has been blocked from entering St. Peter's Square.

The Women's Ordination Conference, led by American priest Father Roy Bourgeois, was stopped by police Monday and told to take down their banners supporting women as priests. When police confiscated the banners, a scuffle broke out, and several of the activists were detained.

The group attempted to deliver a petition to the Vatican, calling on Pope Benedict to lift the ban on female priests. The petition included around 15,000 signatures. But the Vatican has refused to ordain women due to Roman Catholic Church tradition and because Jesus chose only men as his apostles.

In a separate open letter to Vatican officials, Father Bourgeois asked how God's call to men to become priests can be considered authentic if God's call to women is not.