Police arrested and questioned four student activists from Madrid's Complutense University on Friday over an anti-church protest at the institution's chapel.
Demonstrators burst into the chapel on March 10, shouting slogans and stripping off down to the waist, in protest against the "Catholic Church's oppression of women and the lesbian, gay, transgender and bisexual community."
The archbishop's office in Madrid described the protest as "an act of profanity" against the chapel and held a "healing mass" for 1,000 worshippers there on Sunday.
Police are investigating whether the protest constitutes a crime against religious sentiments, which can carry up to six years in prison, but which in practice, according to experts, is usually punished with a fine. The students have since been released.
University professors were last week among those signing a letter in support of the students' right to protest.