A 15-year-old boy was arrested because he was suspected of stabbing a student in broad daylight in front of a mall.
The police said the teenager surrendered to a police station in East London on Friday and was arrested on suspicion of murder. He remains in custody.
The officers were summoned to Broadway in Stratford, East London, shortly after 3:00 pm Thursday after being reported stabbed.
The 15-year-old, whom the police described as Baptista Adjei from North Woolwich on Friday, was attacked in or after getting off a bus near the Stratford mall.
Police and witnesses said that the teenager's friends and passers-by rushed to help after being stabbed in front of a McDonald's in the mall. But the police and paramedics found him "critically injured," and although he was struggling to save his life, he was declared dead at 3.49 pm at the scene.
Baptista's aunt, Aretha Adjei, told the Evening Standard that her nephew was a "fun-loving boy, very popular with everyone in the church," who "would always make you laugh."
A second boy, age 15, was also found stabbed and taken to a hospital in East London, where his injuries were classified as non-life threatening.
Five hours later, officials were called to the Brandon Estate in Camberwell, South London, where they found an 18-year-old man with stab wounds who died an hour later.
The police said the teenager had been chased by a group of men who then hit him and killed him.
The Mayor of London, Sadiq Khan, said he was "devastated" by the two deadly knife wounds and asked the witnesses to come forward. He said, "It is no honor to be silent."
Khan said, "I am deeply devastated to hear that two teenagers have been killed on our streets: a 15-year-old boy in Stratford and an 18-year-old man in Southwark.
"My thoughts are with the relatives of both victims in this incredibly difficult time."
He continued, "The knife crime steals the future of our young people – and it devastates our communities. It has to stop. That is why, despite the government's huge cutbacks, we finance dedicated officials on our streets to fight violence, and focus on tackling the causes of crime and investing in youth and prevention services. "