Officers are to quiz 11 of those held over the murder of an 18-year-old man near the Foot Locker store on Oxford Street at 13:45 GMT on Monday.
The victim died at the scene near the junction with Stratford Place.
Less than five hours later, near Oxford Circus, a man aged 21 was stabbed in the thigh. Three people have been arrested.
Police said it was too early to confirm whether or not the two incidents were connected.
However, Insp Bruce Middlemiss, the duty officer for police in Westminster, said: "They are a similar sort of circumstances, youths, possibly from the same south London area."
The first stabbing happened at the height of what is traditionally the busiest shopping day on Oxford Street.
A post-mortem examination of the victim is due to take place later and his next-of-kin have been informed, but his identity has not been released.
Mim Shaikh, who was friends with the victim, said: "I lived in the same block as him and we went to the same high school. We'd go down to play football together.
"When I'd see him in the street, he'd give me a cheeky grin and a nod."
Det Ch Insp Mark Dunne said there appeared to have been two groups of young people "opposing each other" prior to the fatal stabbing.
He added: "A number of weapons have been recovered from that scene. Whether I have got the murder weapon I don't know.
"There's an assortment of items but no guns.
"This is probably the busiest place in the United Kingdom right now on the busiest shopping day so it's been difficult for us to piece together what has happened and clear this area to do the job we are paid to do and work out a sequence of events here.
"The flipside of that is we have probably more witnesses than we would normally have and I'm very much looking forward to them coming and speaking to us."
The area, close to Bond Street Tube, was closed between Duke Street and New Bond Street and shops within the cordon were forced to close.
Police reopened the area 06:00 on Tuesday, with just a small crime scene cordoned off outside Foot Locker.
The second stabbing took place near to the junction with Regent Street at 18:20.
The 21-year-old victim is in a stable condition and his injury is not thought to be life-threatening.
Three men have been arrested in connection with the attack and are in custody awaiting questioning.
Mr Middlemiss said the public should not feel unsafe shopping in the West End.
"There is a high police presence here, we have a number of operations in place already for the shopping times because we were aware of the amount of people who would be here," he said.
"We are going to go back and review the intelligence for both incidents and see if we need to increase the police presence here, but at the moment we are not concerned for ordinary members of the public who are shopping."