A woman has been stabbed to death in the street by a man who was then run over and killed.
Witnesses tried to distract the attacker in Maida Vale, west London on Monday in desperate attempts to save the victim.
She and the man stumbled into the road during the struggle and he was hit by a car.
Mulugeta Asseratte told the PA news agency: “It is a very terrible incident.
“I was coming up from the bakery to Chippenham Road and suddenly heard shouts of ‘stop it, stop it’ and it was a girl being attacked by a gentleman.
“I tried to look and even say ‘stop it’ and it would not.
“All of a sudden it moved to the street and no sooner than they reached the street than a driver came and hit both of them.
“We were all shocked and shouting and my heart goes out to the families of this fatal incident.
“This is something one sees in movies, not in real life.”
Raad Jiyad, 52, told the Evening Standard that he had tried to distract the attacker despite the fact that he had a large knife.
He told the newspaper: “I saw a man over a woman with a knife, I saw the blade.
“I heard her screaming in pain.
“I wanted to try and make him come towards me instead to buy her some time.
“We tried to save her.
“He had this big knife so no one wanted to go near.
“Then this car came through and ran him over.
“It was all over so quickly.
“He went completely under the car.
“I think the driver was just trying to buy time to distract him.”
The Metropolitan Police said it had launched “an urgent investigation” after the horror unfolded in Chippenham Road at around 9am on Monday.
Ambulance crews were called as well as London Fire Brigade, who freed the man as he was trapped under the vehicle, but both the woman and man were declared dead at the scene.
Detective Chief Inspector Neil Smithson said: “This morning an incredibly serious incident has unfolded in the Westminster area which has seen two people pronounced dead.
“Our officers are on scene and are working to establish exactly what has happened.
“As soon as we have more information will keep the public updated but for now we must avoid speculation and allow investigators to do their work.
“What I can say at this time is that the incident is contained and we do not believe there to be any wider threat to the community.
“We have spoken to a number of people already but anyone else who witnessed the incident, or who has information that could help us, is urged to make contact immediately.”
Police are working to inform their families, and said that post-mortem examinations will be arranged.
A spokeswoman for London Ambulance Service said: “We were called at 9.02am to reports of an incident on Chippenham Road, W9.
“We sent a number of resources to the scene, including two ambulance crews, two clinical team leaders, an advanced paramedic, two tactical response units, a medic in a car, an incident response officer, and members of our hazardous area response team.
“We also dispatched by road London’s Air Ambulance.
“Despite our medics’ efforts, two people died at the scene.”
Witnesses can call police on 101 quoting the reference 1496/24jan, or to remain anonymous contact Crimestoppers on 0800 555111 or online.