One woman is critically ill and three others have been injured after they were stabbed near a supermarket car park in west London.
Police were called at about 10:30 BST to The Avenue in Hampton, near a Sainsbury's branch.
Three women were found injured at the store's car park, while the fourth woman was found at a property nearby.
A 66-year-old man has been arrested on suspicion of attempted murder.
London Ambulance Service (LAS) said the women were treated at the scene for stab wounds to the chest, legs and back.
One of the victims, aged 53, was taken to a major trauma centre by an air ambulance while three others, aged 67, 62 and 71 years old were taken by road to a south London hospital as "priority".
A woman working at the nearby Hampton Pub, who did not want to be named, said the attacker "appeared to be out of it" and was targeting women in the "completely unprovoked" attack.
"He only went for the women.
"The man ran through and started attacking them. It didn't seem like they knew him, it was completely unprovoked.
"If it had been ten or 20 minutes earlier there would have been children there because it's right outside a primary school."
The witness said one of the injured women was a local resident and raised the alarm.
"The woman who was badly stabbed in the back made it up to the police station and raised the alarm," she said.
Sally Caddle, manager of the YMCA youth centre which is based near the scene of the attacks, told BBC News: "I heard a scream and ran out to the car park to see what was happening and heard a young lady saying she'd been stabbed.
"I went to stem the bleeding that was coming from her back. I helped a second woman as well who'd had an injury.
"One of the Sainsbury's staff came out of the store to assist and quite shortly the emergency services arrived."
Ben Brady, a nearby business owner, went outside after hearing a "commotion".
"We are based next to the community police centre and I saw some passerby helping a lady there who had been stabbed.
"They laid her down on floor and she seemed to be trying to catch her breath but couldn't. Then I kept hearing people shouting that there was someone in the car park with a knife, so I ran over there to see if I could help out or stop him from attacking anyone else.
"I then saw another two ladies at the back of their car. They both were suffering stab wounds to their sides and back and they were screaming and pointing 'he's over there he's over there'... After stabbing the ladies, he was just walking down the road waving the knife around."