Army staff are continuing work on an object found on a building site in Bristol's Broadmead, on Thursday - thought to be a World War ll bomb.
The Broadmead area is being redeveloped
Police said there was no immediate danger but say people should listen to radio or TV broadcasts for information.
The incident has been declared a major emergency although police stressed this was just as a precautionary measure to allow contingency plans to be used.
The large metal object was found 11 metres underground at Penn Street.
It is though it could be a wartime bomb although this has not been confirmed by the Army Explosive and Ordnance Disposal (AEOD) teams.
Experts from AEOD have asked for more police officers in the area to ensure resources are available in case evacuation became necessary.
Police said the incident was causing considerable disruption to traffic.
An Avon and Somerset Police spokesman, said: "We would like to reassure people living and working in the city centre that they are not in any danger.
"Should the situation change, we'll put our plans into place and inform people immediately."