A 26-year-old man was being held on suspicion of raping a child who was attacked on her way home from school.
The alleged assault happened in Edmonton, north London, on Friday after the 11-year-old victim stepped off a bus.
The attacker pounced on the girl and dragged her into nearby Jubilee Park, where she was assaulted for up to three hours, police said.
She required surgery for injuries sustained during the ordeal.
Det Insp Simon Ellershaw, said: "This was a horrific and unusual attack of a defenceless schoolgirl making her way home via her usual route, along one of Enfield's busiest shortcuts, at the beginning of Friday afternoon rush-hour."
Police issued a description of the man they said they were hunting for the assault. He was described as black with afro hair.
Officers in Sapphire Command arrested a suspect at an address in Edmonton. He was being questioned at a north London police station.
Ch Insp Taylor Wilson said that support for the victim and her family was being provided.
"Although this is an isolated and extremely rare incident, we are treating this with the utmost seriousness," he said.
"The investigation is being run by a specialised unit from Edmonton Police Station. They are working hard to identify the suspect and offer dedicated support to the victim and her family."