The parents of five-year-old Ashya King were arrested by Spanish police after a member of the public recognised them from a Twitter appeal, police have said.
"Within hours of appealing on Twitter for help to find the boy we received a call from a member of the public who had seen the appeal, which allowed us to locate him and his parents," a spokesman for Spanish National Police said.
British police are due to arrive in Spain today to continue their investigation and question Ashya's parents.
They had requested a European arrest warrant for the couple on suspicion of neglect for taking the boy out of Southampton General Hospital.
The head of the police force which applied for a European Arrest warrant against the parents of Ashya King has now urged their release.
A British mother whose son's life was saved by proton therapy has told ITV that she defied NHS doctors' advice to pursue the treatment.
The parents of Ashya King said they took him out of a hospital to seek proton beam treatment which is not available under the NHS.