Bomb disposal crews were called to Warnham last night after a rusty hand grenade was found near a lake.
The device was spotted by a member of the public near Mill Pond Lake at around 4.30pm, police said.
Officers along with Explosive Ordnance Disposal experts attended and removed the grenade, shutting Warnham Road in the process.
Neighbours said they heard a bang as an explosion was carried out.
One mum said her son was walking their dog on the common at the time. “He was turned back by a policeman who told him there was going to be a controlled explosion of a grenade that had been found,” she said.
Horsham weatherman Ian Michaelwaite, of Netweather, drove past the scene and added: “Police are blocking traffic and an army bomb disposal truck is parked by the weir.”
The Warden @HorshamWildlife tweeted a photo of the bomb disposal crew on site and wrote: “Just your average day as a Nature Reserve Warden! #wardening #Warnham #Horsham”