An actor playing an angel in a production starring Mikhail Baryshnikov collapsed on stage at the Kennedy Center last night, stunning the audience and abruptly halting the show.

The performer, Gregory Mitchell, was taken by ambulance to George Washington University Hospital, where he was being treated last night, authorities said.

The trouble began around 8 p.m., about 25 minutes into the performance of "Forbidden Christmas, or the Doctor and the Patient." The production, an absurdist tale that takes place in Soviet Georgia in the 1950s and weaves together dialogue, movement, music and puppetry, was being performed before a full house of about 500 in the Terrace Theater.

Mitchell, playing an angel in the Kennedy Center's second night of the production, was trying to rescue Baryshnikov's character, who had thrown himself into the sea, witnesses said. At that point, Mitchell began breathing heavily and collapsed on stage, members of the audience said.

Theater managers "brought the curtain [down] and they asked if there was a doctor in the house," said Tiki Davies, a spokeswoman for the Kennedy Center who was not at the performance. Emergency medical technicians and a doctor tended to Mitchell backstage, and the audience was directed to go home, she said.

Davies said it was unclear what caused Mitchell's collapse.

Olive Hopkins, a District resident who attended the performance, said, "We just saw him fall, and we thought that it was part of the performance.

"I thought it was odd when they closed the curtain," she added. "It was too soon to be intermission."

Her friend Helen Anderson of Alexandria said that the production's title, referring to a doctor and a patient, led her to believe that the collapse was part of the production.

"All of a sudden, he was down on the floor," she said. "I hope he recovers."

Produced by the Baryshnikov Dance Foundation, the show takes place in a small town during the Stalinist era, and Christmas has been outlawed.

"A doctor and his mentally ill patient embark upon a perilous and emotional journey" on Christmas Eve, according to a description of the show on the Kennedy Center Web site, and the two address "matters of loss, madness, self discovery and love."

The production, written by Rezo Gabriadze, premiered in Minneapolis in May and has played in cities including Charleston, S.C., Philadelphia and New York.

Davies said the show was sold out for its scheduled week-long run at the Kennedy Center, and she did not know whether the performances would continue.