Shocked visitors ran to help as a man allegedly used a screwdriver to attack a famous Thomas Gainsborough painting at the National Gallery.
The 1785 painting, Mr and Mrs William Hallett, better known as The Morning Walk, was damaged during the incident on Saturday at around 2.15pm.
A spokeswoman for the gallery said two long scratches were carved through the paint of the artwork, but the canvas beneath was not damaged.
Gallery assistants were joined by visitors who rushed to stop the man causing further damage before police arrived.
It led to the evacuation of the east wing, which was closed to the public for around two hours.
A National Gallery spokeswoman said: “The painting was removed from display and examined by the gallery's conservators, who are now assessing next steps.”
The spokeswoman said the damage had been carried out with a "sharp implement", which one witness described as a screwdriver.
Scotland Yard said a man was arrested after the incident on suspicion of criminal damage.
He is currently being quizzed at a central London police station.