According to the American Life League, student protesters were threatened with arrest at the LA Cathedral:

Today’s scene occurred when more than a dozen young people attempted to attend the inauguration ceremonies of Antonio Villaraigosa, mayor-elect of Los Angeles, who is a pro-abortion Catholic. The young adults are part of American Life League’s 2005 Crusade for Life walks in which they are trekking from San Diego to Sacramento spreading the truth about the incompatibility of Catholicism and support of abortion.

The group planned to attend the inauguration ceremonies, beginning at the Cathedral, and peacefully protest with shirts and signs that read: you can’t be Catholic and pro-abortion.

…The young activists were eventually allowed in the Cathedral after the Los Angeles Police Department intervened in the situation. "How ironic that the Cathedral security guards wanted to arrest the students and the LAPD had to step in and point out that such an act would be unconstitutional," said Brown. "The truths of the Church will always stand strong, regardless of who attempts to stifle them…even a cardinal."

I don’t see the part where they were threatened were arrest, but it’s not beyond imagining that the Cathedral security forces would not allow them in.