A 17-year-old youth has been arrested on suspicion of murder after a teenager was stabbed to death in west London on Friday morning, police said.
The victim, also 17, was rushed to a central London hospital, where he was later pronounced dead.
Officers were called to Crown Street following the stabbing in Acton at 11:50am. The teenager was arrested nearby and was taken to a west London police station.
The road has been sealed off while officers from the Homicide and Major Crime Command investigate. Crown Street Surgery was forced to close following the stabbing.
Officers believe they know who the dead youth is, although they await formal identification. Next of kin have been informed.
Detective Chief Inspector Rebecca Reeves, who is leading the investigation, said: "Crown Street would have been a busy at the time of the stabbing and we are appealing for any witnesses, or anyone with any information, that was in or around the area to contact us.
"We are particularly keen to hear from anyone who saw or heard an argument outside a café on Crown Street."
An employee of Acton Pet Stores, which is on the street and was forced to close, said police were "everywhere".
The woman, who did not want to be named, said: "I didn't see anything, but the whole street is cordoned off. It is not good, whatever it is."
She added that the residential street had seen trouble in the past. "Police are often here. Very often."
Any witnesses or anyone with any information are asked to call the incident room on 020 8721 4868, or contact Crimestoppers anonymously on 0800 555 111 or via crimestoppers-uk.org.