An eight-year-old boy reported being sexually assaulted by a stranger in a Blackpool amusement arcade's toilet, police said.
He was approached by the man and coaxed into a cubicle at the Golden Mile Amusements at around 10.30am yesterday, with the attack interrupted by the youngster's mum shouting his name, a spokesman said.
The pervert fled as the mum called in police, and a manhunt was launched.
After a breakthrough, a 25-year-old man from Blackpool was arrested on suspicion of sexually assaulting a child and was still in custody this morning, the spokesman added.
One witness described seeing crime scene investigators and uniformed officers close to the men's toilets at the popular seafront arcade.
The boy cannot be identified for legal reasons.
In a statement, Lancashire Police said: "We were called around 10.30am on Sunday, October 7, to reports of a sexual assault in Blackpool.
"An eight-year-old boy reported he had been assaulted by a man in Golden Mile Amusements on the Promenade.
"The man was allegedly disturbed before making off from the scene.
"Following enquiries, a 25-year-old man from Blackpool has been arrested on suspicion of sexual assault of a child and is in custody."