A work by artist Mark Rothko at the Tate Modern has been defaced with a small amount of black paint.
The London gallery said one of the artist's Seagram murals was defaced about 3pm on Sunday.
Paintings by Rothko, one of America's most important post-war artists, have sold for tens of millions of pounds.
The BBC reports the gallery was shut for a short period and then reopened. Police are investigating the incident.
A Tate spokeswoman said the visitor visitor defaced the mural 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.