Two military grenades and “unidentified chemicals” have been found at a property in Lowestoft, leading to 60 houses being evacuated.
Officers have set up a 100m cordon at Normanshurst Close while they wait for an Army bomb disposal team to arrive.
Suffolk Police made the discovery while executing a warrant just before 13:00 BST. They are being supported by Suffolk Fire and Rescue crews.
A man in his 50s has been arrested on suspicion of possession of a firearm.
People living in the area said they were asked by police to pack a bag before leaving their houses.
Chloe Marie Cullen, 23, had to leave her flat on neighbouring Normanston Drive, with her dog Amber.
“I heard lots of sirens then I saw the police walking up the road towards the house. But I stayed calm as they seemed to have it under control.
“The police told me to pack an overnight bag as there’s a chance we might not be able to get back in this evening.”
Shaun Waters, landlord of the Norman Warrior pub in the town, said many people had come to the pub to seek refuge.
“They’ve been told to go to the sports centre for the night, but a lot are waiting to be picked up by family instead,” he said.
“It was a bit of Blitz spirit at first but now people are starting to think about work tomorrow and not being able to get back to their houses.”