Police said the attack by Asians on the white teenagers at Bartholomew Road in London's Kentish Town at about 1845BST on Tuesday was being treated as a racial incident.
One of the boys was being treated for serious injuries while the other had suffered slight injuries.
A Scotland Yard spokesman said: "At this early stage it appears the victims were attacked by a group of up to seven Asian men in their early 20s."
Witness appeal
The spokesman said two 17-year-olds and a 15-year-old boy were in custody at Kentish Town police station while another 17-year-old was being questioned at Albany police station.
Police are appealing for witnesses to come forward and are not linking it to any previous incident.
Resident Lyn Costello, who lives close to where the stabbings happened, said the Kentish Town community was extremely concerned about Tuesday night's incident.
"It's frightening, we are not used to gangs around here. It's a nice area, everybody really gets on," she said.
"People are really worried because the last thing we need is a backlash and groups of people coming down looking for revenge."
Anyone with information about Tuesday night's incident should contact Kentish Town police station on 020 87336134.