In a video clip posted by Just Stop Oil on social media Reverend Dr Sue Parfitt (82) held a chisel over the case as Judy Bruce (85) hit it with a lump hammer several times. Parfitt then held up a banner saying "THE GOVERNMENT IS BREAKING THE LAW."
The historic document was not damaged and the toughened glass of the case was only slightly chipped, the library said in a statement.
There are only four original Magna Carta texts, the 13th century English treaty which established that nobody was above the law. Two of them are kept in the British Library, one is in Salisbury Cathedral and the other is held in Lincoln Castle.

"As a Christian I am compelled to do all that I can to alleviate the appalling suffering that's coming down the line and is here already ... we must Just Stop Oil," Parfitt could be heard to say in the video clip.
London's Metropolitan Police said later they had arrested two people on suspicion of criminal damage in relation to the incident.
The government passed new legislation in 2023 allowing tougher action against disruptive protests.
(This story has been corrected to clarify that the case was not badly damaged, in the headline and in paragraphs 1 and 3)
Reporting by Muvija M Editing by Hugh Lawson
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