Fire crews have been fighting a large blaze at a workshop on Holyhead Marina.
An eyewitness said "30ft high flames" were coming out of the roof of a marine engineering workshop after reports of an "explosion" on social media.
North Wales Fire Service had four crews tackling the fire on Anglesey, which started at about 21:10 BST on Thursday.
Ynys Mon MP Albert Owen tweeted there were "no reported injuries" and the area around the Holyhead RNLI station was evacuated as a precaution.
"Emergency services have the matter under control," he added.
Clywed am y ffrwydrad yng Nghaergybi heno.— Rhun ap Iorwerth (@RhunapIorwerth) August 23, 2018
Hearing there are no injuries relating to the #Holyhead marina explosion. Do hope so. Thanks to all emergency services attending.
Holyhead Port, one of the UK's busiest ports with daily ferries to the Republic of Ireland, said the fire was not affecting its operations.
The Welsh Ambulance Service said it had been made aware of the fire but paramedics were not required.
The fire service confirmed the blaze had been brought under control by 23:30.
Vicki Owens, of Holyhead RNLI, said volunteers were attending the station's AGM just an hour before the fire broke out at the workshop next door.
"The windows blew out with the heat, there's some smoke damage and there's muck everywhere, but it's not too bad," said Ms Owens.
"When I got here the flames coming out of the roof were 30ft high and could be seen from far away. It was shocking."
Witness Allan Johnson said on Twitter: "Heard large bang and went outside to check, could see billowing black smoke coming from the marina area near to the RNLI base.
"Went to our attic and took first video. Could see blue lights and heard more arriving. Walked down to front and took second video."
RNLI Holyhead also tweeted: "Devastating scenes here in Holyhead. Please stay safe everyone."
Devastating scenes here in Holyhead 😢 Our ILB has been taken to safety but this is awful to see. Please stay safe everyone . pic.twitter.com/t5PSqr2jXw— RNLI Holyhead Lifeboat (@HheadLifeboat) August 23, 2018
People living nearby were advised to keep their windows and doors closed and stay away from the area.
This latest incident at the marina comes months after boats were smashed during Storm Emma in March.