A vandal who walked into the Tate Modern art gallery and defaced a valuable painting by Mark Rothko is being hunted by police.
The visitor daubed black paint on the mural piece on Sunday afternoon, a spokeswoman for the gallery said.
Tate said it does not have a price for the defaced piece, but paintings by the Russian-born artist often fetch tens of millions of pounds.
The gallery was shut for a short period and then reopened following the incident at 3.25pm.
Scotland Yard confirmed it had launched an investigation into the matter and is yet to make any arrests.
The force said it was looking for a white male in his late 20s.
Eyewitness Tim Wright (@WrightTG) posted on Twitter: "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."
The Rothko was defaced with words that appeared to read:"Vladimir Umanets, A Potential Piece of Yellowism."
A picture he uploaded to the social networking website showed five or six words scrawled on the bottom-right corner of the piece, with black streaks of paint running down from the daubed writing.
The Tate spokeswoman added: "There was an incident at Tate Modern 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.
"The police are currently investigating the incident."
Earlier this year, Rothko's Orange, red, yellow was sold for £53.8 million - the highest price ever paid for a piece of post-war art at auction.
The 1961 painting went under the hammer at Christie's in New York.
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