Detectives investigating the murder of teenager Hani Abou El Kheir in Pimlico are urging people to come forward with information one week after the attack.
Hani, 16, was stabbed to death last Sunday by a group of people believed to have been wielding knives and swords.
Police were called to Lupus Street at 6.50pm, and Hani was rushed to King's College Hospital where he later died.
Last night four males were arrested on suspicion of murder and one male and three females on suspicion of assisting an offender.
They were being held in custody at various police stations, Scotland Yard said.
A week after the attack detectives have returned to Lupus Street on the edge of Pimlico, one of London's most affluent suburbs, to redouble their efforts in the hunt for his killers.
Officers are appealing directly to the community of Churchill Gardens Estate, where Hani lived, to come forward and help with their investigation.
Leaflets are being handed out to residents, including an appeal from Hani's mother, Pauline Hickey, with whom he had an "unconditional and unbreakable bond", and ones from other members of his family and his girlfriend.
Chief Superintendent Paul Rickett said: "We're here in numbers to try and see if there's anybody else we haven't already spoken to who has information to help us bring the people who did this savage attack to justice.
"Our thoughts remains with Hani's family at this dreadful time, but I'd like to reassure the local community that has been affected by this that we're doing all we can."
He added: "I want to send a really clear message that if you're thinking about providing refuge or assisting people that we are looking for in relation to this horrendous attack - if you are thinking about helping them in any way - then you too face arrest and prosecution."
Police believe at least 10 people were involved in the incident and are keeping an open mind regarding a motive.
A post-mortem examination on Tuesday found Hani died from multiple stab wounds.
The arrests last night bring the total number arrested to nine. On Thursday a 20-year-old man was arrested in north London on suspicion of murder and bailed until late March.
Witnesses or anyone with information is asked to contact the incident room on 020 8345 3715 or Crimestoppers on 0800 555 111.
