Watch a video report by ITV News Anglia's Rob Setchell
Police have warned there is a "real possibility" a Second World War bomb discovered on a building site could explode, as they outlined their plans to make it safe.
People living within 200m of the site have been ordered to evacuate, amid warnings that any blast could cause "shrapnel injuries and damage buildings".
Bomb disposal experts are currently building a sandblast wall around the device found at Bollard Quay in Great Yarmouth, which is predicted to take until 5pm.
A 400-metre perimeter has been set up around the device after it was found by a contractor working on the construction of a new river crossing late on Tuesday morning.
Supt Nathan Clark from Norfolk Police told ITV News Anglia some people were still staying at home, despite police warnings.
"We are now delivering a much harder message: you must go," he said.
"If that bomb goes off you're looking at shrapnel injury, you're looking at potential damage to local buildings and infrastructure, so there is a real risk now."
He said bomb disposal teams would have to cut the bomb in order to do a safe controlled detonation, and had been building defences around the bomb.
"That means they've had to build quite a substantial sandblast area around the bomb because there is quite a real possibility that it could actually go off," he said.
He warned that "even with that sand blast wall that still packs a real punch".
"It's not often we find something of this magnitude right in the middle of our town, right in the middle of our community. This is unprecedented," he added.
As a result of the impact to the community, agencies in Norfolk declared a major incident on Tuesday night.
Anyone refusing to leave the 200m inner cordon would delay the operation to defuse the bomb, he said.
How will army bomb disposal teams will detonate the Great Yarmouth bomb?
They have built a mitigation barrier - to minimise the impact of any explosion - around the 250kg bomb since it was discovered on Tuesday;
Experts will then cut into the bomb which could take up to 12 hours before carrying out a controlled explosion;
If this is successful, the remainder of the bomb will be moved out to sea for a further controlled explosion.
An emergency helpline - 01493 330369 - has been set up for people who need help with evacuation from the exclusion zone.
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