Hundreds of people were unable to return to their homes after a suspected explosive device was found in a suitcase, police said.
Around 100 homes were evacuated in the Southsea area of Portsmouth after officers were alerted to the device and ammunition found in the bedroom of a house in Jessie Road.
A Hampshire Police spokesman said the suitcase is thought to have been there for a number of years.
Royal Navy teams inspected the contents and the decision was made to set up a 100-metre cordon, he said.
Residents who are unable to return to their homes have been directed to a rest centre nearby on Devonshire Avenue, set up by Portsmouth City Council.
Chief Superintendent David Peacock said: "Public safety is paramount and we're taking the measure of evacuating homes as a precaution while the Royal Navy team can move in to make the area safe as soon as possible.
"The ambulance service is on hand to help with any vulnerable people and Hampshire Fire and Rescue Service is also on the scene."
He said it was unclear when people would be able to return to their homes.
