Saturday's bomb in a racially mixed neighborhood market sent nails flying.
Police say the call could be a hoax, but they are investigating all possibilities.
Officials say the call was made from an area where a black teen-ager was killed 1993. That killing prompted a national outcry and allegations that the city police force is riddled with racism.
No one was killed, but Scotland Yard said four people had been seriously injured in the explosion near Brixton Road and Electric Ave. at 5:25 p.m. local time.
"Clearly, the person who set this off is disturbed and had no regard for human life," said Commander Hugh Orde, the Metropolitan Police officer leading the investigation.
George Jones, 42, a merchant at the street market, said he moved the device before it exploded.
A utility bag "was handed to me by somebody who said someone had forgotten their shopping. I unzipped it and opened it wide, and I could see what it was," said Jones, who described it as being wrapped with tape and nails.
"I picked it up and moved it 10 to 15 yards and put it by a brick wall to stop people getting hurt. ... Then it went off. I was blown across the road, and a couple of nails lodged in my leg, and I was hit by concrete and glass."
Jones said he had telephoned police before the explosion.
Brixton, one of London's most ethnically diverse areas, was torn by rioting in 1985, but was the scene of a giant street party in 1996 when South African President Nelson Mandela visited.
"On a Saturday, you can't find anywhere that's more heaving with people than Electric Avenue, with all the market stalls and the shops," said Andrew Scott-Bolton, who lives near the market.
"There was a huge explosion. Most of us thought it was thunder, but it was a very, very loud bang," he said.