A teenage boy was stabbed to death in broad daylight in central London, Scotland Yard said.
Officers found the wounded 17-year-old less than three miles away from Parliament.
He was treated at the scene and taken to hospital, London's Metropolitan Police said in a statement. He later died from his wounds.
“This awful attack took place in broad daylight in a busy part of central London," said detective chief inspector Andy Partridge. "A teenager with his whole life ahead of him has been taken away from his family who have been left heartbroken."
Appealing for information, he added “We know that lots of people were in the area at the time and may well have seen what unfolded. We need them to do the right thing and get in touch right away with what they saw along with any images or moving footage captured before, during or after the attack.
“There is a college nearby and many young people would have been on their break. We are keen to hear from anyone with information that can help us build a clearer picture of what took place and bring those involved to justice."
His superior officer, Superintendent Andy Britain added that the force was "horrified by this attack" and would do "everything in our power to apprehend those behind this appalling crime."
He added that extra officers would be patrolling the streets in the area around where the stabbing took place.
The force said the teenager's next of kin had been informed but formal identification was yet to take place.
His death follows a spate of killings in the capital.
A woman was stabbed to death in Camden, north London on Sunday evening, minutes before another man was shot to death less than a mile away in Kentish Town.
A man was also shot earlier in the day, although eyewitnesses suggested he may have killed himself when a bullet he fired ricocheted off a car and hit him.