Luke Sundberg and three of his friends were in line inside the Louvre in Paris on Sunday, waiting to pose for a photo in front of the Mona Lisa, when they heard gasps.
A man dressed as a woman had sprung from a wheelchair and ducked under a rope barrier separating the painting from the crowd of about 100 people.
The visitors watched in disbelief as he began pounding on the glass that shields the painting. Then, Mr. Sundberg said, the man smeared what appeared to be cake all over the glass protecting what is one of the world’s most recognizable pieces of art.
“I was in awe,” said Mr. Sundberg, 20, a first-year student at Colby Community College in Kansas. “It’s something so historic that seems untouchable.”
The protester, whom officials have not named, faked a disability to get close to the Mona Lisa, according to the Louvre. The painting was not damaged, museum officials said.
Mr. Sundberg said he and his friends posed with the painting after the glass was cleaned and then they began to leave the museum. On their way out, he said, they saw the man, who looked straight at Mr. Sundberg as he recorded him with his phone.
“He threw a whole bunch of roses at me and then he started yelling,” he said.
Videos on social media showed that the man, speaking in French, yelled that there were “people who were destroying the planet” and “that’s why I did it.”
The Mona Lisa, painted by Leonardo da Vinci in the 16th century and perhaps the crown jewel of the Louvre’s collection, is typically swarmed by camera-wielding tourists. The painting is held behind a thick glass case, an effective shield against pastries.
After the man smeared the glass, he was tackled by security guards, Mr. Sundberg said.
The Louvre said in a statement that officials with the museum had followed its usual procedures for people with reduced mobility, “allowing them to admire this major work of the Louvre.”
Once he was near the painting, the man threw the pastry that he had hidden, the museum said.
Security guards seized the man and escorted him out before handing him over to the police, the museum said. The museum filed a complaint, officials said.
There have been several attempts to vandalize the painting, some more successful than others. In 1956, a man threw a stone at the painting, shattering a glass shield and scratching Mona Lisa’s left elbow, causing a chip of paint to fall off.
The man initially said he had no real reason to commit the act.
“I had a stone in my pocket and suddenly the idea to throw it came to my mind,” the police quoted him as saying. He later said he was jobless, had no money and simply wanted to be jailed during the cold weather.
Steve Keller, a museum security consultant who has worked with the National Gallery of Art, the Smithsonian Institution and other museums, said such episodes happen “more often than we hear about.”
Vandalism “is one of the top three concerns we have,” he said, adding, “We do take steps to prevent it, but it’s not really predictable.”
Mr. Keller, who has worked in museum security since 1979, said he used to see cases like this once a year at the Art Institute of Chicago. He has found lipstick on paintings and handprints on portraits, and he once stopped a man from painting a piece of art that he claimed was his and not finished.
Mr. Keller said that many museums did not want to place paintings in protective glass because it diminished the experience for museum visitors.
But the Mona Lisa is a predictable target, Mr. Keller said. In 1911, it was stolen by three Italian handymen and recovered 28 months later. In the 1950s, a visitor attacked it with acid. In 2009, a woman threw a teacup at its glass.
People who engage in such stunts usually just want to get on television, said Stevan Layne, the founding director of the International Foundation for Cultural Property Protection and a former director of security at the Denver Art Museum.
He said that such acts of vandalism carried out by demonstrators have nothing to do with the issues they are trying to call attention to. “They’re not really accomplishing anything,” Mr. Layne said.
Claire Fahy contributed reporting.