UK police arrested two people on Wednesday after environmental activists sprayed an orange substance on Stonehenge, the renowned prehistoric UNESCO World Heritage Site in southwest England.

The Just Stop Oil protest group said a pair of its activists had "decorated Stonehenge in orange powder paint" to demand that the UK's next government legally commit to phasing out fossil fuels by 2030.

Footage posted on social media showed the activists, wearing "Just Stop Oil" branded T-shirts, spraying at least two of the megalithic monuments with the orange substance from a small canister.

The group said on social media they had used "orange cornflour" and claimed it "will soon wash away with the rain".

Wiltshire Police said in a statement it had "arrested two people following an incident at Stonehenge this afternoon".

"Officers attended the scene and arrested two people on suspicion of damaging the ancient monument.

"Our inquiries are ongoing and we are working closely with English Heritage," the police added, referring to the public body that cares for hundreds of the country's historic places, including Stonehenge.

The incident comes in the middle of the UK's general election campaign, ahead of voters going to the polls on July 4.

  'Disgraceful' 

 It drew immediate condemnation from Prime Minister Rishi Sunak, who called it "a disgraceful act of vandalism to one of the UK's and the world's oldest and most important monuments".

"Just Stop Oil should be ashamed of their activists," he added.

Just Stop Oil said it had chosen to stage the action on the day before the Summer Solstice festival, when crowds gather at the site to celebrate the northern hemisphere's summer solstice.

A spokesperson for the group said that although the Labour party, which is widely expected to win next month's election, has vowed not to issue any new oil and gas drilling licences, "we all know this is not enough".

"We have to come together to defend humanity or we risk everything. That's why Just Stop Oil is demanding that our next government sign up to a legally binding treaty to phase out fossil fuels by 2030," the spokesperson added.

"Failure to commit to defending our communities will mean Just Stop Oil supporters... will join in resistance this summer, if their own governments do not take meaningful action.

"Stone circles can be found in every part of Europe, showing how we've always cooperated across vast distances -- we're building on that legacy."