Dozens of people are spending the night in a rest centre as Army disposal experts work to defuse an unexploded Second World War bomb.
The bomb was discovered by construction staff near Aston, Birmingham, on Monday morning and it has been identified as a German shell weighing up to 500lbs (227kg).
Police closed the A38 Aston Expressway and nearby homes and businesses within a 500-metre cordon were also evacuated after the alarm was raised at 9.45am.
The Red Cross said around 80 people were being cared for overnight by volunteers.
Birmingham Police Commander Chris Johnson, said: "An initial assessment from Army bomb disposal suggests this is a large explosive device − that's why an extensive cordon has been put in place for public safety.
"We don't take decisions like this lightly but public safety has to be our number one priority.
"Hundreds of homes have been evacuated and we are working with our partners to provide emergency accommodation.
"There have been some difficulties in accessing the device given the terrain and its position within a construction site − but all agencies involved are working as quickly as possible to bring the situation to a safe conclusion with brave military staff risking their safety to ensure that of the public."
Police added they expect the A38 to be closed for most of Tuesday, with 250 tonnes of sand brought in as part of the "major, delicate" operation.