A memorial to German philosopher Karl Marx has been attacked with a hammer and "will never be the same again", a charity has said.
The Friends of Highgate Cemetery Trust, the charity which runs the cemetery in north London, said the marble plaque on Marx's grave appeared to have been attacked with a hammer over the weekend in what looked like "a deliberate attack on the memory of Karl Marx himself".
Ian Dungavell, the trust's chief executive, said: "His name has been singled out, although the person wasn't particularly co-ordinated."
The "deliberate and sustained attack" on the Grade I-listed monument, which puts it on a par with the most important buildings in the country, was "very upsetting", he said.
"It will never be the same again, and will bear those battle scars for the future.
"It's already scarred with traces of paint and previous damage.
"I'm really cross about it because it's a particularly inarticulate form of protest and it's not going to win any fans.
"On a human level, I'm upset when anyone destroys a grave. To do something so mindless is particularly upsetting."
The history of the marble plaque was "quite complicated", Mr Dungavell added.
It was first used on the grave of Marx's wife Jenny von Westphalen in 1881 and was moved when the remains of Marx and his wife were exhumed and reinterred in a more prominent location in the cemetery in 1954, after the original grave was "not felt to be a suitable marker for someone of such international significance".
The monument is owned by the Marx Grave Trust, which is represented by the Marx Memorial Library in Clerkenwell, which will make any decisions about future repairs, Mr Dungavell said.
But he added that the monument's Grade I-listing means "you don't rush in to do these things quickly".
"We will be seeking specialist advice."
A Met Police spokesman said: "Police were called at 17.12 on Monday February 4 to reports of criminal damage to a memorial in Highgate Cemetery, N6. No arrests have been made.
"Initial inquiries have been completed and at this stage the investigation has been closed. If any further information comes to light, this will be investigated accordingly."