A woman was taken into custody after throwing a shoe at Hillary Rodham Clinton as the former US secretary of state began a Las Vegas convention speech.
The incident happened after Clinton took the stage before an Institute of Scrap Recycling Industries meeting at the Mandalay Bay resort on Thursday.
Clinton ducked, and she did not appear to be hit by the object. She then joked about it.
"Is that somebody throwing something at me? Is that part of Cirque de Soleil?"
Many in the audience of more than 1,000 people laughed and applauded.
Brian Spellacy, US Secret Service supervisory special agent in Las Vegas, said the woman was being questioned and would face criminal charges.
In the hotel hallway, the middle-aged blonde woman sat calmly on a sofa. She said she threw a shoe and dropped some papers, but did not identify herself to reporters.
Spellacy and Mark Carpenter, spokesman for the recycling institute, said the woman was not a credentialled convention member and was not supposed to have been in the ballroom.
Clinton said she felt politics today led people to "do what they think will be rewarded".
A piece of paper apparently thrown by the woman appeared to be a copy of a Department of Defence document dated August 1967. It referred to an operation "Cynthia" in Bolivia.
During a speech on Tuesday, Clinton said she was seriously considering a presidential bid.