Stornoway Coastguard were called by a number of people concerned that up to 12 red distress flares were seen in the Sound of Barra between South Uist and Barra at about 9.20pm yesterday.
Two coastguard teams and a rescue helicopter were sent, and fishing boat responded to an emergency broadcast for assistance.
But rescuers found the flares were set off from a beach party at Eolaigearraidh, near Barra Airport.
Martin Collins, Stornoway Coastguard watch manager, said: "This was an extremely irresponsible action as it has resulted in several rescue units being called out for no good reason.
"Not only that but while assigned to search for the source of these flares, the rescue units were not available for anyone who may have genuinely needed their help. It is also prohibited under the Merchant Shipping Act to set off red distress flares if you are not in difficulty and this incident has been reported to the police.
"The public should ensure that all marine flares are disposed of correctly and you can contact your local coastguard station for information about flare disposal. Under no circumstances should marine flares be disposed of in a manner such as this."