A police search was launched after youths with shotguns were seen firing in an area where children play.
Police warned anyone carrying firearms and using them illegally that they face arrest.
Residents in Wivenhoe heard gun shots coming from the port area at about 9pm on Tuesday night.
Two young men were spotted in Bath Street carrying what appeared to be rifles into a derelict dock building where children often congregate.
The Essex Police helicopter was scrambled in a bid to trace the culprits.
Wivenhoe beat bobby PC Andy Bannerman said: "We treat any call like this seriously and went out to investigate immediately. Nothing was found in the area directly around Bath Street so we called in the police helicopter.
"Once we were all happy that no men carrying rifles were around the area the search stopped but we still need more information to be able to clear up this matter."
Det Sgt Roger Napier, of Colchester police, said: "We do not think the weapons were air rifles because of the shots heard.
"Trespassing on land with a firearm is a criminal offence. And to go into an area where children play is very dangerous."
Anyone with information should contact PC Bannerman at Wivenhoe police station on 01206 822082 or Colchester police on 01206 762212 or Crimestoppers on 0800 555111
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