The blast was the seventh in four days in the capital. A man, aged 23, was treated in hospital for slight head injuries and released.
The sergeant spotted the bomb in a telephone box near the high-security Paddington Green police station, where terrorist suspects are interrogated. He cleared the area minutes before the blast destroyed the box and blew out nearby windows.
Minutes before, after the device had been spotted, a warning was given to a telephone exchange that a bomb would go off in Paddington.
Six other people have been injured, none seriously, in earlier blasts, which have caused limited damage but considerable traffic disruption. There were two explosions in north London on Friday night. The IRA has admitted planting both devices and said it was also responsible for the Paddington blast.
Commander George Churchill-Coleman, head of Scotland Yard's anti-terrorist squad, said: 'Miraculously, no one has been murdered or seriously maimed, but it is no thanks to the IRA, whose cynical use of misleading and deliberately imprecise coded warnings is designed to confuse the emergency services.'