As the Linz church newspaper reported on Tuesday, the Italian police asked the group to pack a poster with the words "Equality" on it.
The police also collected the activists' ID cards and handcuffed two people who were unable to identify themselves and led them away from St Peter's Square.
According to the group, the poster campaign was intended to campaign for equal rights for women, lay people and LGBTQ+ in the church.
The English abbreviation LGBTQ primarily stands for non-heterosexual people who identify as lesbian, gay or queer.
Variations are LGBTQI, LGBTIQ+ or LGBTQIA+. Each letter stands for a different sexual orientation or gender identity.
According to one participant, other groups were allowed to keep their posters.
The police justified this by stating that they had been granted permission.
According to the Lateran Treaty, the Italian police are responsible for security in St Peter's Square. They act at their own discretion.
An enquiry by the Church newspaper about the incident remained unanswered, according to the editorial team.