"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 in an e-mail.
Museum-goer Tim Wright witnessed the act of vandalism and posted an image on Twitter.
The 23-year-old Bristol resident said 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."
‚ÄúIt was very surreal. It wasn‚Äôt something we expected to see. One minute he sat down, and the next minute he put his foot over the barrier,‚ÄĚ Wright said of the vandal.
Wright said they saw the man as he finished up the tag and then made his getaway. 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.
Here is a look at some of the stories that CNN plans to follow this week:
Biden, Ryan face off
Vice President Joe Biden and Republican vice-presidential nominee Paul Ryan will duke it out Thursday in the first and only vice-presidential debate. The showdown takes place at at Centre College in Danville, Kentucky.
A recent CNN/ORC International Poll found that 55% of likely voters thought Ryan would fare better in the debate, while 39% said Biden would win.
Biden and Ryan face off eight days after President Barack Obama and GOP candidate Mitt Romney met on stage. Obama and Romney will debate again on October 16.
Romney to deliver major foreign policy address¬†
Mitt Romney will deliver a major speech on foreign policy Monday in Virginia, an aide to the Republican presidential nominee said last week, as bloodshed continued in Syria and fresh questions surfaced about the Obama administration's handling of last month's consulate attack in Libya.
The speech, which a source said would take place at the Virginia Military Institute in Lexington, comes as foreign policy has taken a larger role on the campaign trail.
Romney has criticized Obama for his dealings with Iran and China. Immediately following the consulate attack, Romney accused Obama's administration of sending "mixed signals" on American values. He was roundly criticized, including by some Republicans, for injecting politics into a dangerous international situation. Since his initial statements, new developments have called into question the administration's handling of security at the Benghazi consulate, both before and after the attack.
Somalia's new president has named a prime minister as the nation sets up an effective central government following decades of civil war.
Abdi Farah Shirdon, an economist who ran an import business in neighboring Kenya, was named prime minister Saturday.
President Hassan Sheikh Mohamud, a political newcomer who has worked for several international organizations, assumed leadership last month.
"The appointment of a new prime minister is another important step forward in the country's peace building process," said Augustine P. Mahiga, the top U.N. envoy to the nation.FULL STORY