A group climbed on to the landmark in central London on Friday night and hung banners from the famous dome.
Police were called at 4.20pm to reports that four people were on the stone gallery area of the dome of the cathedral, a spokesman for City of London Police said.
Four people came down at separate times, with the last late on Saturday night, and each was arrested on trespass and criminal damage charges.
There were no road closures or cordons in place during the protest, police said.
A video live-streamed on Facebook showed protesters, who were wearing harnesses, looking down on the crowds gathered below.
A post alongside the video states: "We are climbing St. Paul's Cathedral to protest at the shocking state of fathers' rights in the UK.
"Roughly 200 children are taken away by the family courts every day from one or both parents. This MUST stop."
