A painting by artist Mark Rothko at the Tate Modern has been defaced by a small amount of black paint, a spokeswoman for the gallery has confirmed.
The London gallery said one of the artist's Seagram murals was defaced at about 15:25 BST.
Paintings by the Modernist painter, one of America's most important post-war artists, have sold for tens of millions of pounds.
The gallery was shut for a short period and then reopened.
Police are investigating the incident.
On Twitter, eyewitness Tim Wright said: "This guy calmly walked up, took out a marker pen and tagged it. Surreal.
"We gave a description to the gallery. Very bizarre. He sat there for a while then just went for it and made a quick exit."
A Tate spokeswoman said: "Tate can confirm that there was an incident in which a visitor defaced one of Rothko's Seagram murals by applying a small area of black paint with a brush to the painting."
Russian-born Rothko emigrated to the US at the age of 10, and went on to become an important post-war abstract expressionist.
He was commissioned to paint the Seagram murals in 1958 for Manhattan's Four Seasons restaurant, but they were never installed.
Shortly before his death in 1970, he presented some of the murals to the Tate Gallery.
In May, Rothko's Orange, red, yellow was sold for $86.9 million (£53.8m) when it went under the hammer at Christie's in New York.
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