A man has claimed responsibility for defacing a valuable Mark Rothko at the Tate Modern art gallery, but denied he is a vandal.
Scotland Yard has 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 piece appears to read: "Vladimir Umanets, A Potential Piece of Yellowism."
Today 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.
"The main difference between Yellowism and art is that in art you have got freedom of interpretation, in Yellowism you don't have freedom of interpretation, everything is about Yellowism, that's it.
"I am a Yellowist. I believe what I am doing and I want people to start talking about this. It was like a platform.
"I don't need to be famous, I don't want money, I don't want fame, I'm not seeking seeking attention.
"Maybe I would like to point people's attention on what it's all about, what is Yellowism, what is art?
"It's good people are shocking about what happened, no-one is realising what actually happened, everyone is just posting that the piece has been damaged or destroyed or defaced.
"But I believe that after a few years they will start looking for it from the right angle. So that's why I did it."
"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.
Scotland Yard has confirmed it has launched an investigation into the matter. The force said it was looking for a white male in his late 20s.
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.
Mr Umanets, who would not reveal his age or where he lives, said he knows he is likely to be arrested, but added: "I believe that from everything bad there's always a good outcome so I'm prepared for that but obviously I don't want to spend a few months, even a few weeks, in jail. But I do strongly believe in what I am doing, I have dedicated my life to this."
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."
Yesterday the Tate Modern was shut for a short period and then reopened after the incident at 3.25pm.
The gallery 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.
A spokeswoman said: "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."
Eyewitness Tim Wright (@WrightTG) posted on Twitter: "This guy calmly walked up, took out a marker pen and tagged it. Surreal.