World War II bomb exploded unexpectedly in a Norfolk city, police said.
Emergency services and agencies declared a major incident after the discovery of the large unexploded device at a river crossing in Great Yarmouth on Tuesday, and had been working to disarm it.
On Friday afternoon, Norfolk police said there was an “unplanned” blast, but no one was injured.
Army specialists had cut through the bomb using a technique that causes the explosives to burn slowly and burns that material.
Officers had said there was a risk of an accidental explosion.
Following the explosion, the force said on Twitter: “We can confirm that the World War II unexploded bomb has exploded at Great Yarmouth.
“This was not a planned explosion and happened during slow fire work to defuse the explosives.
“All military and emergency services personnel are accountable. We will provide you with more information as soon as we have it.”
Work to cut into it began on Thursday, but the water it took to do the job reduced the effectiveness of the sand barrier around the device.
The device is about a meter long and weighs about 250 kg and was discovered by a contractor working on the third crossing of the River Yare.
There was a barrier 400 meters long on site.
Nick Davison, Assistant Chief Constable of Norfolk Police, said: “This has been a laboriously long process, but public safety and that of the people involved in the operation has been at the heart of the decision making.
“This was the final stage of a delicate operation that has caused a lot of disruption in the city, but we hope that this can be resolved quickly and that the cordons can be lifted if everything goes according to plan.”
A spokesperson for Cadent, which manages the local gas grid, says: “Our team has carefully inspected our nearby pipes. There is no indication that the explosion caused any damage to our assets and the gas continues to flow safely.”