Visitors to a Chicago food court got more than what they ordered on Monday afternoon. Their lunches were interrupted by a flash mob led by opera soprano Renee Fleming and cellist Yo-Yo Ma. The famous singer and musician joined Chicago high school students and members of the Chicago Symphony Orchestra in what seemed like an in impromptu performance at the Thompson Center.

Cellist Yo-Yo Ma and and opera singer Renee Fleming perform with a choir of dozens of high school students. (Kiichiro Sato/AP)

Ma and Fleming performed a piece by Rachmaninoff and led a sing-along of “America the Beautiful.” The performance was in support of the Chicago Symphony Orchestra’s Citizen Musician initiative and the Lyric Opera of Chicago’s Renee Fleming Initiative, which fosters the building of community through the arts.

Some of the people who caught the performance had received an e-mail heads-up from the CSO (which kind of defeats the purpose of a flash mob, yes?) reports Chicago’s WBEZ. But for many, the show was a surprise.

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Yo-Yo Ma, who was chosen to receive Kennedy Center Honors in 2011, is something of a flash mob pro: He’s also performed in other flash mobs with DePauw University and the Chicago Children’s Choir.

Ma and Fleming speak to the crowd during their performance. (Kiichiro Sato/AP)