Google MapsHome owners in Newtownards, County Down, have been asked to leave their homes following the discovery of a suspected World War Two bomb.
It was found at a building site at the Rivenwood housing development in Movilla Road on Friday.
Ulster Unionist Party councillor Pete Wray said he understood 450 households were affected.
On Saturday evening, the Police Service of Northern Ireland (PSNI) issued a leaflet to residents requesting them to leave their homes by 10:00 BST on Sunday due to the “possibility of risk to life and property”.
The statement added that all homes and businesses within a 400m (0.25 miles) radius of the “sizeable” munition were impacted and that the operation could take “five or more days to complete”.
OtherAt the scene
BBC News NI’s Darran Marshall in Newtownards
A police cordon is in place at the top of Rivenwood Road, where new houses are under construction.
All morning a steady stream of cars have been leaving the development.
Residents say they’ve been advised to leave their homes by 10:00 BST and told they could be out for five days.
A bomb disposal unit is currently on site. Army personnel are using diggers to pile sand on top of the device ahead of a planned controlled explosion.
Most of the residents I spoke to say they are going to stay with family, including newlyweds Andrew and Sara Lightbody, who got married just a fortnight ago.
They told me a police officer called at their door last night and told them to evacuate by 10:00 BST. They’ve packed their dogs and a few personal items.
James Fraser is the developer.
He told BBC News NI the suspected device was discovered yesterday by workers during some ground works for a new phase of the development where 40 homes are under construction.
‘Significant disruption’
Police have asked the public to avoid the Movilla Road area and said that updates will be provided in due course.
“I understand the significant disruption this will cause to the community,” said Supt Johnston McDowell.
“Our priority is to keep residents and the local community safe.”
The Ulster Unionist councillor told BBC News NI: “This is a very complex and unusual thing to happen.
“It was explained to me that this is only the second of this scale in the UK. Something similar happened in Plymouth.”
The Ards Blair Mayne leisure centre is being set up as an “emergency support centre” for affected residents and it will be open from 09:00 BST.
