Emergency services were called to the Fernways unit on Cecil Road, Ilford, at about 22:20 BST on Saturday to reports of an assault.
Two people found with stab wounds were pronounced dead at the scene.
A 50-year-old man was arrested on suspicion of murder and is in custody at an east London police station.
Police believe the victims and the man arrested knew each other, but said they were not related and were not staff at the unit, which is part of the care provision of Redbridge Borough Council.
Officers say they know the identities of the deceased and their next of kin have been informed.
Post-mortem examinations on the pair will be held later, the Met said.
A woman in her 50s and a man in his 80s have been stabbed to death at a sheltered housing unit in north-east London.