A second man has been arrested in connection with the fatal knifing of 14-year-old Martin Dinnegan.
Martin's mother said the killing was "horrendous"
Martin was stabbed to death during an attack by a group of youths in Islington, north London, on Tuesday.
Officers described the murder as a "vicious attack carried out in daylight on a busy street".
The man, 20, was arrested at an address in Islington. A youth, 15, who was arrested on Wednesday, has been remanded until Saturday.
The 20-year-old is being held at an east London police station.
The arrest came as Metropolitan Police announced a special operation to tackle violent offenders across London over the summer months.
LONDON'S MURDER VICTIMS IN EIGHT DAYS
19 June - Sian Simpson, 18, stabbed in Croydon
22 June - Carlos Eduardo Segove, 23, shot in Acton
23 June - Mikey Brown, 23, stabbed in Kingston
23 June - Annaka Keniesha Pinto, 17, shot in Tottenham
23 June - Ben Hitchcock, 16, stabbed in Beckenham
26 June - Martin Dinnegan, 14, stabbed in Islington
27 June - Abu Shahin, 18, stabbed in Ilford
Martin's death is the latest in a series of killings involving young people in London.
On Saturday, 16-year-old Ben Hitchcock from Penge, south London, was stabbed to death in a fight involving up to 40 youths in Southend Road, Beckenham.
Fourteen teenagers were arrested in connection with the incident and were later released on police bail pending further inquiries.
Ben's murder followed the shooting of a 17-year-old girl during a fight at a bar in north London in the early hours of Saturday morning.
Annaka Keniesha Pinto died in hospital after being shot in the Swan bar in Philip Lane, Tottenham.
Three people were arrested in connection with the death and later released on bail.