Protesters who have occupied a section of a huge natural gas pipeline being laid from Milford Haven to England say they aim to stay "for the long haul".
Protesters have occupied machinery at the site and the pipeline itself
Around 14 people are inside the pipe at Trebanos in the Swansea Valley and another is occupying a crane.
The pipeline will carry gas from two liquefied natural gas (LNG) terminals in Milford Haven to Gloucestershire and supply a fifth of Britain's gas.
The National Grid said work had stopped on that section of the pipeline.
The pipeline, which will stretch 150 miles from south west Wales and through the Brecon Beacons into England.
The protesters moved onto the construction site above the village of Trebanos early on Monday, some climbing onto machinery and others crawling inside the pipeline.
They said they had been invited onto the land by the landowner Eira Wyn Jones.
Mrs Jones said: "They asked if they could come, I said yes and 'the gas' asked me, and I knew I couldn't say no..because they'd come anyway. It makes life exciting doesn't it?"
One protester, identified as "Nick", described the conditions inside the pipe.
"You can kind of walk down it but very crouched over. It's normally best to slide down it, because it's downhill, down the mountain and then try and walk back up it on your knees.
Protesters say the pipeline is "not ecologically friendly".
Asked how long they were prepared to keep up the protest, Nick said: "Well, we're very determined people, and if it's a case of willpower, I'm sure we've got a lot more willpower than someone who doesn't really care and they're just here to be paid - to pay their mortgage - so longer than them"
The campaigners are questioning the safety of the project.
Jim Dunckley from the Safe Haven Network said: "This was a forest at one point in time and it was reclaimed farmland and it's been completely ripped up and devastated.
"We want to highlight the ecological damage that the National Grid are doing to the environment, not just along here, but along the pipeline route."
Mr Dunckley said there were also climate change issues.
"This is a fossil fuel, it's unsafe, not ecologically friendly and the government should be investing in renewable energy - wind, wave hydro power and things like that," he added
In a statement the National Grid said the Milford Haven-Aberdulais natural gas pipeline which received ministerial consent a year ago was "vital in providing the UK with secure supplies of gas".
"Construction sites can be dangerous and we are very concerned about intruders on our site both for their safety and welfare," the statement went on.
"National Grid always works hard to listen to the concerns of local people and we are more than happy to keep talking with them when the protest ends.
"Our work has been suspended, and we hope to discuss the position with the landowner at the earliest opportunity."
South Wales Police said around 14 protesters were taking action.
"Officers will remain at the scene to maintain the peace of the protest and to ensure the health and safety of all concerned," a spokesman added.