Protesters from Bristol, Cardiff, Oxford and Bath all chained themselves together outside Aberthaw Power Station in the Vale of Glamorgan.
Police said the the protest caused traffic tailbacks.
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Campaigners issued a statement saying: "While we might be technically breaking the law today, we are really trying to uphold it.
"The Government is committed to emissions reductions and if we are to meet those targets we cannot go back into the coal age.
"Clean coal technology is 20 years away at least, and we don’t have time to waste in addressing climate change.
"We need to phase out coal and replace it with renewable sources of energy. Numerous technical studies have shown it is possible to power the UK without fossil fuels."
The power station is owned by RWE npower, which also runs Didcot Power Station.
(CD/JM)
