The incident happened at Oslo Cathedral in Norway, where the two environmental activists stripped off their clothes and started kissing on the floor.
The protesters were campaigning for the 'F**k for Forests' group, who believe in public sex to outrage people and raise awareness of deforestation.
The pair had to be held apart while one churchgoer called the police. A third person - also involved with the group - was placed under arrest when he was caught taking pictures of the scene.
A police spokesperson said: "We arrested three people for sexual activity in front of the altar in Oslo Cathedral."
The unidentified trio accepted a fine, with one saying: "We had to [either] pay a small fine or spend 16 days in jail.
"We chose the former, because we didn't want to spend so long without having sex."
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