A massive unexploded World War II bomb discovered in an east London river was defused by a specialist army team overnight, police said.

The discovery of the one tonne device, said by police to be "the largest bomb ever to be dropped on London" forced the closure of several rail services and part of the Underground.

The bomb was dredged from a river by a mechanical digger near Bromley-by-Bow Underground Station in east London on Monday, during work to clear a site for the 2012 Olympic Games.

Royal Engineers finally made the "Hermann" bomb safe, a day after it began to tick during efforts to defuse it, the Metropolitan Police said.

London Underground and some national rail services were suspended and a 200-metre exclusion zone set up.

An air exclusion was also in place as the location is below the flight path to City and Heathrow airports.

Some 380 tonnes of sand were packed around the device to minimise any blast.

To add to the risk, the site was next to a gasworks a sewage works.

Police Superintendent Gary Buttercase said he would recommend the bomb disposal officer for a medal.

"This isn't like you see where you've got a remote control," he said.

"This is a human being dealing with a bomb of unimaginable magnitude, the largest bomb ever to be dropped on London."

East London was heavily bombed during World War II.

The Royal Engineers bomb disposal squad are the modern-day equivalents of teams set up during the conflict to deal with unexploded Nazi ordnance.

- AFP