HVIDRA called on citizens to "peacefully defend the Church against a shameless insult."
Riot police are securing the protest.
Outside the Cathedral, about 50 believers are praying and holding a banner with the image of the Virgin Mary.
The Zagreb Pride organisation convened its rally outside the Cathedral because, according to an announcement on its Facebook page, "over the past month the Catholic Church has been on an unprecedented homophobic campaign" consisting of scandalous, insulting and hateful statements systematically demonising the LGBT community.
An association for the promotion of family values called on citizens and believers to ignore the Zagreb Pride rally called "Love thy neighbour outside the Cathedral."