UK Police hunting the partner of the grandmother of 12-year-old Tia Sharp said that he had been arrested on suspicion of murder.
A Scotland Yard spokesman said that Stuart Hazell, 37, was arrested in a public place at 8.25pm in the south London borough of Merton after he was identified by a member of the public.
Earlier a manhunt was launched after police hunting for Tia found a body at the house where she was last seen alive in New Addington, near Croydon, south London.
The body was found at the home Hazell shared with Tia's grandmother Christine Sharp. The body has not yet been identified.
Scotland Yard had warned the public not to approach Hazell.
The announcement that a body had been found came just a few hours after police began a forensic search of the terraced house.
A police spokesman said then: "We are seeking to find Stuart Hazell to be interviewed in connection with this case.
"He should not be approached and, if seen, people should call 999 immediately.
"There have been no arrests in connection with this investigation at this time.
"The family of Tia Sharp has been informed of the discovery."
Hazell was interviewed by police as a witness on Wednesday but then released.
Mrs Sharp, who left her home with police shortly before today's search began, told reporters this morning that her boyfriend was out helping in the hunt for Tia.
She said that Tia's mother Natalie was "in bits" and she did not know where she was as "she needed to get away".
After she left, a cordon was placed around the house on The Lindens and white-suited forensics officers were seen going inside.
Until then, journalists had been able to approach the house, although at times reporters were met with hostility from family members.
Police did not reveal where the body was discovered.
Officers were seen taking a ladder into the property this afternoon, sparking rumours that it may have been found in the loft. A dark holdall was later seen being removed from the house.
Between 80 and 100 officers had been involved in searches for Tia, including in woodland near the house.
But the house was not fully searched until today.
Scotland Yard had also been trawling through hundreds of hours of CCTV footage but found no images of her after she was reported missing.