The 45-year-old woman and her 23-month old daughter were discovered after officers were called to an address in Oswald's Mead, Hackney, east London, shortly after 9am today.
A 53-year old man - understood to be in a relationship with the female victim - was found injured and has been arrested on suspicion of murder.
The victims were named locally as Val Forde, a youth support worker, and her daughter Jahzara.
It is understood the man arrested by police lived at the property. He was taken to an east London hospital where he remains in a stable condition with injuries which are thought to be non-life-threatening, police said.
Scotland Yard said the Directorate of Professional Standards had referred prior contact with the victim to the Independent Police Complaints Commission.
A number of schoolchildren left floral tributes near the scene, as police cordoned off the area and forensic teams entered the property.
Prayers were said later for Ms Forde and her daughter at a service at the Wally Foster Community centre in Hackney.
Both victims had not been formally identified and post-mortems examinations were due to take place, a Metropolitan police spokesman said.