More than 100 firefighters are tackling a fire at the Afan Lido in Port Talbot after a blaze broke out at the pool.
South Wales Police said the fire was "significant" and closed roads because of the amount of smoke.
Some houses have been evacuated and residents have been told to stay in and keep windows and doors closed because of potentially harmful chemicals.
The fire at the lido in Sandfields broke through the roof and fire crews were unable to enter the building.
Police said the evacuations were undertaken as chemicals at the pool could have been affected by the fire. They are not treating the fire as suspicious.
Residents in Channel View have been evacuated from their homes, and the council has provided emergency accommodation for those who may need it.
Tywyn Primary School on Channel View will be closed on Thursday but will re-open on Friday.
Seventeen fire vehicles including three aerial appliances were at the scene and firefighters were expecting to be tackling the fire through the night. There have been no reports of casualties.
Lifeguard Justine Jones, 19, was one of four staff on duty at the poolside and was the person who raised the alarm when she saw fire break out from underneath a fake rock feature.
She told BBC News Online: "I spotted the fire in the poolside. There is a feature temple and fake rocks and the smoke was coming from inside the rocks.
"I heard a 'pop' and there was smoke and flames everywhere. It was coming from the bottom by the slides."
Ms Jones, who has worked at the lido for two-and-a-half years, said there was only one person swimming at the time but it took them a moment to attract his attention.
"We whistled at him and said, 'there's a fire, quick, run!'"
They checked the changing area before leaving. She added: "All the staff got their heads together and acted fast."
Ms Jones said the building had fortunately been very quiet at the time the fire started.
Her car keys and personal belongings are still inside the building, and she had to leave her car parked outside.
Mid and West Wales Fire and Rescue Service was first called to the leisure centre in the Sandfields estate at 1650 BST.
The Princess Margaret Way was closed in both directions between Dalton Road and Victoria Road and police were directing traffic away from the area.
South Wales Police said a number of other surrounding streets were also closed and motorists were being advised to avoid the area.
Neath Port Talbot councillor Mike James, cabinet member for leisure, is also the chairman of Celtic Leisure, the company which runs the building for the council.
He told BBC Wales: "This is a 53-year-old iconic building on the seafront, and seeing the damage that's been done is quite frightening.
"We have already decided whatever happens here tonight, whether this can be restored to its former glory or whether it'll be rebuilt, we can promise you now that the people of Aberafan will get back a leisure centre in the style of the Afan Lido in the near future."
He said firefighters had praised the quick response of staff in evacuating the building so rapidly.
BBC Wales reporter Tom Singleton said by early evening the fire was so intense it was preventing the firefighters from entering the building, and there were reports of explosions from within.
He added: "The roof of the building is glowing white hot, making it unsafe for those firefighters to enter."
He said the manager at the lido had praised the speed with which his staff had evacuated the building and had called it a "worrying time" for everyone concerned with the lido.
The lido houses a swimming pool, spa, and the Aquadome, which has water slides and tyre rides, and is also a music venue. The band lostprophets were due to play there in March.
Brian Ireland and his wife were having Christmas lunch in the Water Margin opposite the Afan Lido and had to be evacuated from the restaurant.
They saw ambulances arriving before taxis ferried him and other people from the site.
Mr Ireland said: "We were in a Christmas do and we noticed smoke, and people coming round in aluminium heat sheets and ambulances coming.
"Next thing we were all being evacuated and put into taxis and taken back as far as the Apollo Theatre.
"I saw flames and smoke coming from the rear and that seemed the location of the worst lot of fire."
We asked for your comments on this story. A selection have been published below.
Absolutley gutted!!!! the heart of Neath Port Talbot is gone. The only thing that made living in NPT worthwhile. Another aspect of our town torn away by a simple fire. These facilities are vital to the town, so much hard work has been put into the activities that take place in the Lido and was hoping to start martial arts training sessions soon there. I hope with a little luck, and the support of the community, it'll be back up, and running again.
Adam Khan, Cadoxton, Neath Port Talbot
Never seen such thing in my life been down the lido twice now! my mams car is stuck in the lido car park the smoke is black and toxic so no one can get their cars and the slides have all calapsed into the pool! Just been down now and there are fire men on the roof still trying to put the fire out! and the police told my mam that it might take a few days to put this fire out :O
Hope Glover , sandfieds port talbot
I have just left the Afan Lido and seeing the amount of black smoke coming from the roof, I am so glad nobody has been hurt. Secondly, I applaud the workforce at the Afan Lido, I know them all well and after speaking to a few, one of their concerns was the swimming club and the other clubs and how will this affect everyone.
As one of a team of volunteer swimming teachers with Aberavon Swimming Club based in the aquadome, it's a devastation for the community and surounding areas living by the sea, we may once again lose our swimming pool. I hope not and hope something can be done. I also hope council members keep the people informed on what is going to happen, in particular the water-based clubs and all the other clubs that use the facilities at such a wonderful and historic complex. It is a must that we keep this life skill of swimming available in our community and for our community(Port Talbot.) It's for likes of rhyan and aimee, who have commented above, who I know very well and for children like my young grandson Ben, that we must be guaranteed a swimming and sports complex.
Finally, I wish everyone at the Afan Lido well and hope jobs and welfare will be safe.
Peter, Port Talbot
I go to the lido mostly every day and was ment to go to the gym and swimming at 4 today. Gladly somthing came up. I loved going to the lido and will miss it very much. I hope no one is hurt and that some how be fixed!!!!
Hannah -jane, little warren, port talbot
Im so upset, its like a part of me has gone, im almost in tears here.
Afaq shah, Port talbot
Come on Neath Port Talbot Council in conjunction with the private company who operate the lido -this is a well loved, needed and indeed used facility in NPT- it is critical to the community so act fast and get re-opened ASAP!!! PS Respect to the firefighters who are fighting as I type to salvage the building.
Grew up in Sandfields lived there for 26yrs.. Spent many a summers in the Lido....Done badminton there for a while too..Gutted to say the least!!! That's all that was left for the kiddies. x
Lucy, Glynneath West Glamorgan
Really devastated to hear about the Lido, my children swim there and also go to gymnastics. Its a huge loss for Port Talbot. I hope the damage isn't as bad as I'm hearing.
Lisa, Baglan, Port Talbot, South Wales
I am shocked to hear the news of the fire! I worked as a secretary at the Afan Lido for many happy years until 1971. I am very sad to think it may now have gone forever, just like so many other lovely old historical buildings in Aberavon and Port Talbot. :-(
Jennifer Lewis Haakonsen, Brynna, Wales
Its terrible! hardly anything left of it! and its still burning! can see everything from my house and the smoke is rediculous! hope everyone is safe!
Needs, Port talbot
Working in england ,had a text from my sister in swansea telling me to phone my wife because she lives in port tolbot and that there was a big fire ,just found out its the lido , all my kids go there in the swimming club , my children all watching on bbc news and are very upset. only there last week for the christmas swimming club party.. gutted
richard peploe, middlesex , england
I am really sad! This is where I go for my swimming club :-(
Aimee Tossell, Port Talbot
oh my god i cant go swimming for years now
rhyan peploe, port talbot wales
i live within 100 metres of the lido and the smoke is everywhere. can anyone tell me how badly it is burnt
matthew, Aberavon, Port Talbot
Just swam there this morning. Big loss for the community.
Gareth Evans, Cornelly
The smoke and light from the aquadome can be seen from up here ( about 15-20 miles away ). The smell is being carried across the air towards us. Hope everyone down there is safe
This is a disaster for the local community and the council with the loss of a site which attracts hundreds of people every day. What will become of the employees at the centre??? How will the council cope with the loss of revenue that the Lido creates??
I hope the situation isn't as bad as feared and that the centre and the pool will be up and running again soon.
Ian Curtis, Port Talbot
Really sad, a great resource in a deprived area- we take our daughter swimming there every week. It will be missed. I sincerely hope it was an accident and not arson. Let's hope it gets rebuilt and reopened and doesnt fall fould of budget cutbacks.
alun hemington-gorse, porthcawl
Gutted. It's the last building I thought would ever catch fire - many happy swimmy memories there. This is a tragedy for SW Wales :(
Alice Prior, Llanelli, Carmarthenshire
I can't believe this has happened. This place was one of the icons of the area.
Daniel Von Schnit Edwards, Port Talbot (from germany)
My son was in football training at the time the fire broke out but the team got out safe. My car is outside the reception, I hope there isnt to much damage. I have never seem so many fire engines in 1 place.
Amy, port talbot
I can't believe it. The fire is huge. The smell is terrible and there us smoke everywhere. I think the Lido will have to be rebuilt.
Debra G, Port Talbot
Theres hardly anything left of it!!!!
sam, port talbot
I couldnt believe when i heard it was on fire :O:Sn cant get over it and im in the swimming club :O
megan muldoon, port talbot
Me and my mum Wendy were sitting down and we heard a helicopter coming, my friend came and told us that it was a fire. We didn't know what it was until she told us, her cousin, the head lifeguard was very brave by getting every one out. We are very afraid and we are told to keep all windows and doors closed. We don't know what to do or if we are being evacuated because we live right next to the Aqua Dome. Mam [ Wendy ], me, and my friend Kate saw the fire and it is very big, we are stuck in doors but hopefully every one is safe.
Sarah Doyle, Port Talbot
The Lostprophets are due to play a gig there in 2 months! Wonder what's gonna happen to that!?
I am shocked! i cant believe that the lido has caught fire, its quite sad :(
Ben Sawyers, Port Talbot
live 5mins from lido, fire engines still going down. Can see a fair amount of smoke.
Vicky Dykes, port talbot w.glam