An estate in Feltham was on lockdown after a woman and young child were stabbed.
The woman, in her 30s, and a one-year-old child are in serious conditions in a west London hospital.
Residents reported seeing more than 20 police cars, London's Air Ambulance and a police helicopter near Hounslow Road.
Witnesses said a police helicopter was spotted flying overhead as well.
A Metropolitan Police spokesman said: "At the scene a woman and an infant had suffered stab wounds.
"Both victims were taken to a west London hospital where their conditions are serious but stable at this time.
“We await further updates. A crime scene has been established and officers remain at the scene."
Officers have not made any arrests in connection with the stabbing.
A neighbour told GetWestLondon: “I live right where it happened. We have been told by police to stay in and lock doors as a man is still on the loose.
“The estate is taped off, and there is an armed response present.”
“At the scene a woman and an infant had suffered stab wound”Met Police spokesman
A London Ambulance Service spokeswoman said: “We were called to reports of an incident in Hanworth.
"We sent an air ambulance and a number of other vehicles.
“We treated two people and took them to a major trauma centre as a priority.”