The police helicopter was called to Borehamwood last night to search for a man thought to have a weapon on him.
A call came into police at 7.11pm on Sunday to reports of a man believed to be in possession of a weapon in the Organ Hall Road area.
A number of police units were dispatched including armed officers and the police helicopter, which hovered and circled across much of the town for some time.
Hertfordshire Constabulary said a "significant search" was carried out but no one was found.
Police have not said what type of weapon the man allegedly had nor given a description of the person they were looking for.
Detective Constable Robert Wood said: "We take reports of this nature extremely seriously and enquiries are currently ongoing.
"Anyone with information that could assist us is asked to report it online at herts.police.uk/report, speak to an operator in our Force Communications Room via our online web chat at herts.police.uk/contact or call the non-emergency number 101, quoting crime reference 586 of 14 March."
Or contact Crimestoppers anonymously on 0800 555 111 or by visiting crimestoppers-uk.org.
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