A Russian artist who claims to have defaced a Mark Rothko mural in the Tate Modern gallery says he has engaged in a piece of art and improved the value of the multimillion-pound work.

The Metropolitan police art squad is investigating an incident at the museum in which a man inscribed a number of words in black ink in a corner of Rothko's 1958 canvas Black on Maroon, before quickly leaving the room.

A visitor to the museum who posted a picture on Twitter of the defaced canvas said that he saw the man sitting quietly in front of the painting beforehand.

"We heard the sound of a pen, but by the time we turned around he was pretty much finished with his tag," said Tim Wright, who was with his girlfriend on a weekend visit to London from Bristol. "The pen ink then just dripped down the painting. Once we realised what had happened, we went to find a member of staff. They were really shocked when they came and saw what he had done."

In the the picture posted by Wright, the black ink on the canvas appears to read: "Vladimir Umanets, a potential piece of yellowism."

According to an online manifesto, yellowism is an artistic movement run by two people named Vladimir Umanets and Marcin Lodyga.

On Sunday night, a man who identified himself as Vladimir Umanets, and answering a phone number provided for an exhibition of "yellowism", said he was responsible for the incident at the Tate Modern and had done so in order to draw attention to what was going on in contemporary art.

"I believe that if someone restores the [Rothko] piece and removes my signature the value of the piece would be lower but after a few years the value will go higher because of what I did," he said, comparing himself to Marcel Duchamp, the French artist who shocked the art establishment when he signed a urinal and put it on display in 1917.

"I was expecting that the security at Tate Modern would take me straight away, because I was there and I signed the picture in front of a lot of people. There is video and cameras and everything, so I was shocked."

"I didn't destroy the picture. I did not steal anything. There was a lot of stuff like this before. Marcel Duchamp signed things that were not made by him, or even Damien Hirst."

He said that he admired Rothko, describing him as one of the great figures in art of the last century, but added: "I don't believe that what I have done is criminal. If the police are going to arrest me, then they are going to arrest me. I am OK with that."

Scotland Yard said on Sunday night that they were looking for a white male in his late 20s.

The gallery said in a statement: "Tate can confirm that at 15.25 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."

The museum was closed for a short time after the incident.

The canvas, one of a number by Rothko owned by the Tate, was in a room with several other works painted by the Russian-born artist, who emigrated to the US at the age of 10 and went on to become one of America's most important postwar artists.

His work commands huge prices. In May, his Orange, Red, Yellow was sold in New York for $86.9m (£53.8m) – the highest price ever fetched by a piece of contemporary art at auction.

The Seagram murals were painted by Rothko in 1958 for Manhattan's Four Seasons restaurant, but they were never installed. He presented a number of them to the Tate gallery shortly before he died in 1970. This year, Tate Modern opened a new Rothko Room as part of its rehung permanent galleries.

Art lovers made their feelings known on Twitter, scrutinising the image of the defacement. "I am a naturally peaceful person, but I wouldn't be that upset if 'Vladimir' accidentally met with a baseball bat," said one.