Experts will start examining the bomb when the 100m zone is clear
The device was found by workers on a building site in Brentor Road, in the Prince Rock area of the city.
It is sticking about six inches (15cm) out of the ground and is 10ins (25cm) in diameter.
Bomb disposal experts have confirmed it weighs about 250lb (113kg). It is thought the device will be removed from the site and detonated at sea.
Specialist equipment has been sent to Plymouth from Wales and when the bomb is moved, buildings on the route may also have to be evacuated.
Emergency planning
"Evacuations are being carried out of properties within 100m [328ft] of the scene," a police spokesman said.
"Properties within the 100m-300m [328-984ft] zone are being advised to open windows and draw curtains."
Some people unable to get into their homes when they returned from work have said they are worried about their pets.
Plymouth City Council's emergency planning officer is at the scene and the council has taken residents with no alternative accommodation to Stoke Damerel Community College.
The Explosive Ordnance Disposal (EOD) team will not begin to examine the bomb until the 100m zone has been fully cleared.
The operation is expected to last for several hours.
Location of Plymouth within England
The discovery of an old World War II bomb in a construction site has caused at least 1000 residents in Plymouth located in South-West England, to be evacuated.

"Evacuations are being carried out of properties within 100m 328ft of the scene. Properties within the 100m-300m 328-984ft zone are being advised to open windows and draw curtains," said a spokesman for the police department in Plymouth.

Workers on the site discovered the bomb at about 10:30 a.m. local time in Plymouth, England on Brentor Road.

Reports say that the bomb is sticking out of the ground by about 6 inches, and weighed an estimated 113 kilograms, or 250 pounds, but could have weighed as much as 500 pounds.

"The item was protruding about six inches from the ground and was described as being up to 10-inches in diameter," said a spokesman for the Devon and Cornwall Police.

The Army bomb disposal team from Wales is currently trying to figure out what to do with the bomb and it could take several hours before authorities can dispose of it. Reports say that the bomb will be taken to the ocean to be detonated.

"The precise nature of this operation and any potential route are yet to be confirmed," added the spokesman.