Residents and traders have been evacuated from nearby buildings, and a 400m cordon has been erected in the area, the Metropolitan Police said in a statement.
A Britain's Got Talent studio was also evacuated after the bomb was uncovered by builders near Wembley Stadium in London.
The 50kg device, thought to date from the early 1940s German air raids on London, was found on Thursday.
"This bomb is a live munition in a potentially dangerous condition so it's important that people listen to the police and evacuate their homes if asked.
The bomb also threatened the Football League play-off finals, which are due to begin tomorrow at the stadium.
In March a 250kg (550lb) bomb was found in Bermondsey, south-east London. It was safely defused and taken to Kent to be destroyed.
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