Police are hunting for a vandal defaced a painting by Mark Rothko at the Tate Modern art gallery.
Eyewitness Tim Wright said: "This guy calmly walked up, took out a marker pen and tagged it. Surreal."
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.
They appear to read: "Vladimir Umanets, A Potential Piece of Yellowism."
More top news
Controversial columnist Katie Hopkins has described her fear that she may die during brain surgery to cure her epilepsy.
A funeral is to be held outside in the US state of Georgia for Bobbi Kristina Brown, daughter of the late Whitney Houston and Bobbi Brown.
Experts have warned that Ash dieback's impact on millions of trees could have serious consequences for the landscape and wildlife.