A painting by Mark Rothko has been defaced at the Tate Modern art gallery.
Police were called after a small area of the mural piece was painted on Sunday, said a spokeswoman for the gallery.
"Tate can confirm that at 3.25pm this afternoon, 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," said the spokeswoman.
The gallery was shut for a short period and then reopened.
Tate said it does not have a price for the defaced piece, but Rothko's paintings often fetch ten of millions of pounds. Earlier this year, his 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.
Scotland Yard said they were looking for a white male in his late 20s. No arrest has been made and the investigation is continuing.
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."
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.