A man has admitted defacing a multi-million pound Mark Rothko painting at the Tate Modern art gallery, but denied he is a vandal.
Scotland Yard launched an investigation after a man was seen daubing black paint on the mural piece yesterday afternoon.
The writing on the bottom-right corner of the artwork appears to read: "Vladimir Umanets, A Potential Piece of Yellowism".
Mr Umanets, who is originally from Russia, said he had written on the painting, but insisted his aim was not to destroy or deface it.
"Some people think I'm crazy or a vandal, but my intention was not to destroy or decrease the value, or to go crazy. I am not a vandal," he said.
Mr Umanets, who studied art, is one of the founders of "Yellowism", which he describes as "neither art, nor anti-art".
"Yellowism is not art, and Yellowish isn't anti-art. It's an element of contemporary visual culture. It's not an artistic movement.
"It's not art, it's not reality, it's just Yellowism. It can't be presented in a gallery of art, it can be presented only in a Yellowistic chambers."
He said he did not plan exactly which painting he would write on, but thinks he found "the perfect choice", and said he feels he may have increased the value.
"To be honest, I do believe I increased the value, it seems probably ridiculous for someone but I do believe in this, I didn't decrease the value, I didn't destroy this picture, I put something new."
The gallery confirmed police were called after a small area of the mural piece in the "Rothko Room" was daubed with black paint or marker.
A spokeswoman said: "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."
Tate Modern on London's South Bank - the most visited art gallery in the world - was evacuated and shut for a short period.
The 1958 painting, Black On Maroon, was one of a series known as the Seagram murals that were gifted to the Tate by the artist in 1969.
Tate said it does not have a price for the defaced piece, but Rothko's paintings have fetched tens of millions of pounds.
Earlier this year, his Orange, Red, Yellow (1961) was sold for £53.8m at Christie's in New York - the highest price ever paid for a piece of post-war art at auction.
Scotland Yard said they were looking for a white male in his late 20s. No arrest has been made and the investigation is continuing, as conservationists assess the damage to the work.
Eyewitness Tim Wright posted a picture of the damaged work on Twitter and wrote: "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 painting was displayed in a gallery covered by CCTV and police are analysing video footage.
The work was part of a series commissioned from Rothko for the Four Seasons restaurant in New York's Seagram building - but it was never installed.
In 1969, the artist donated nine of the paintings to the Tate on the condition that they be displayed in an "immersive environment".