Police received an anonymous call reporting that a man had been seen with a gun near to Pryzm night club in Watford town centre at 12.30am.
Officers immediately rushed to the scene and apprehended four men.
Then seizing a firearm and a vehicle.
Detectives are now investigating the incident in Watford are appealing for witnesses and information.
Detective Inspector Jason Finnegan, from the Watford Local Crime Unit, said: “We are working hard to unpick the circumstances around these events and I appreciate that news of this may be unsettling to the wider community, however at this stage we believe it was an isolated incident.
“Thanks to the person who bravely made the initial report, we were able to immediately locate the suspects and seize a weapon so I would like to thank them for their actions.
“However since then we have made several attempts to contact that person, but the number they provided us is no longer connecting.
“Unfortunately they terminated the call before the member of staff in the force communications room had chance to record any further detail.
“Therefore we are very keen to speak to them, or anyone else who may have seen a man acting suspiciously in the area around Gaumont Approach yesterday evening, as we believe they will have vital information that could assist our enquiries.
“I would also like to speak anyone else who believes they have details that could prove useful to our investigation to please get in touch as soon as possible.”
call our non-emergency number 101 quoting ISR 12 of 13 August.