Insp Gareth Hammett, from Devon and Cornwall Police, said the ordnance was a suspected unexploded WW2 device.
He added the police endeavoured to “resolve this incident as quickly as possible”.
“We do not underestimate the impact and the inconvenience… but we’d also like to reassure the public that we’re working quickly and closely with our partners to try and bring this incident to a conclusion.”
He said approximately 1,200 homes were being evacuated and the Royal Navy ordinance disposal team were on site surveying the device.
He said it was “unsettling” and added “I know it’s really inconvenient and we know the impact”.
“This is not the first time we’ve dealt with an incident like this, as you know, in Plymouth…
“This is all about public safety.”
There were more than 50 bombing attacks on Plymouth between 1941 and 1944 killing 1,178 civilians in the area. Official records estimate that at least 2,820 bombs fell on the city with about 10% failing to detonate.