It is estimated that 13,000 priests, religious (nuns and brothers) and lay leaders were killed by the leftists in the 1930s.
In the “Republican” zone, no Catholic worship was permitted and churches were desecrated and destroyed.
The leftists would go into convents and drag the nuns out and kill them and they would attack seminaries and kill all the students. They attacked individual parishes and Catholic schools and priests, and priests had to hide out and be smuggled around from family to family. Naturally, the entire family would be killed if it was found that they were hiding a priest.
“In the “Republican” zone, no Catholic worship was permitted and churches were desecrated and destroyed.”
In the Basque country the Church was protected (since they fought for independence, not so much for communism); that was the reason the Vatican didn’t officially recognize the Franco government until that area was secured in 1937/early 1938: at that point no religious services were permitted in the remaining “government” territory.