Nine-year-old Karen Hadaway and Nicola Fellows, 10, were found dead in bushes in Wild Park, Brighton, East Sussex on 10 October 1986.
The young friends, who lived nearby, had been sexually assaulted and strangled.
The girls had been reported missing the day before and their disappearance led to a huge search involving friends, family and scores of police officers.
On the 20th anniversary of the unsolved case, the victims' relatives took part in a march to mark the milestone, clutching teddy bears and single roses.
The suspect, who cannot be named for legal reasons, was detained by officers from the Surrey and Sussex major crime team after new evidence came to light.
No one has ever been successfully prosecuted for the murders.