(CNN) -- A Mark Rothko painting was defaced at London's Tate Modern on Sunday.
"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," a museum spokeswoman wrote to CNN in an e-mail statement.
Museum-goer Tim Wright witnessed the act of vandalism and posted an image on Twitter.
The 23-year-old Bristol resident told CNN he was at the Tate during a day trip to London with his girlfriend when the couple realized what was going on. He noticed a man walk into the exhibit, but he thought nothing of it until he heard a "smashing sound."
"It was very surreal. It wasn't something we expected to see. One minute he sat down, and the next minute he put his foot over the barrier," Wright said of the vandal.
Wright said they saw the man as he finished up the tag and then made his getaway. He said he and his girlfriend stayed at the exhibit while a group of nearby women went to find museum staff. An alarm soon went off, and the museum was evacuated. Wright said he and his girlfriend gave a description of the event to a museum employee.
Tate confirmed that the gallery was "briefly closed" due to the incident.
"It's just not the thing you expect to see in an art gallery," Wright said. "I've never seen anything like it. It's quite shocking, actually."
Kate Rothko Prizel and Christopher Rothko, the artist's children, said: "The Rothko family is greatly troubled by yesterday's occurrence but has full confidence that the Tate Gallery will do all in its power to remedy the situation."
"Our father donated his legendary Seagram paintings to the museum in 1969 sensing the commitment of the institution to his work and impressed by the warm embrace it had received from the British public. We are heartened to have felt that embrace again in the outpouring of distress and support that we and our father have received both directly and in public forums."
Rothko, a Russian-American abstract expressionist, was commissioned to do a series of paintings for the Four Seasons restaurant of the Seagram building in New York in 1958. Though he started the series of murals, he famously reneged, deciding the swanky New York restaurant wasn't an appropriate home for his art.
Rothko rejected the commission, but completed paintings stemming from the project, many of which made their way into the halls of museums, including the U.S.'s National Gallery of Art. They have been on display in many of Tate's locations.
"The large dark works which form the core of this exhibition came to Tate from the painter Mark Rothko as a result of lengthy negotiations begun by Sir Norman Reid, then Director of the Gallery, with the artist in 1965," reads a description on Tate's website.
The murals arrived in London in 1970 -- the year Rothko committed suicide.