A man carrying a plastic ball bearing (BB) gun sparked an armed police hunt for more than two hours in a quiet village.
Police officers carrying machine guns descended on a little road in Rustington, near Littlehampton, after a woman alerted them to a man carrying a weapon.
But officers said a description from a witness suggested the man was only carrying a BB gun that shoots plastic pellets.
Eight officers did a sweep of the area after the alarm was raised from Richmond Close just before 10am yesterday morning and the hunt was not called off until after midday.
Inspector John Merrick, division head at Chichester Police, has now issued a warning about the use of imitation guns.
He said: "We got a report from a member of the public who had seen a male walking down the street with what she believed looked like a gun, although she described it more like a child's toy.
"Under the circumstances we can't take a chance and we have to respond accordingly in case it was a firearms incident.
"No threats were made to anyone and the object wasn't pointed at anybody but we had to contain the area and make checks. We are still trying to trace the man.
"When we find him we can establish what type of gun it is but even if it is an imitation gun we can still arrest him under the Firearms Act.
"More legislation is planned in the future to prevent the use of imitation guns. The information we have suggests the man was carrying a BB gun that fires little plastic pellets.
"It is right that the lady contacted us as it could have been a more serious incident, you never know.
"People should not be out in public carrying guns of any type because it could lead to their arrest and prosecution.
"We will follow this up with further investigations and hopefully gain enough information to make a prosecution."
Inspector Martin Pattenden, of Chichester Police, added: "We get lots of these types of calls, especially in areas where there are lots of kids around and they carry imitation guns.
"Often if we believe it is an imitation there will not be any need to send out a firearms response unit, or armed officers, but it is a judgment call and depends on the situation."
A resident of Richmond Close said: "I saw about four police cars and have just been told by the police that it was because there was a man carrying a BB gun.
"I don't like plastic guns and I won't let my kids play with them. Guns kill people and I don't think people should carry them even if they are imitation."
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