The antics led the state's second highest-ranking official, Lt. Gov. Peter Kinder, to denounce the performance in a tweet Sunday. He said it was "disrespectful" to the president.
"We are better than this," the Republican tweeted.
State Fair officials said the show was "inappropriate" and "does not reflect the opinions or standards" of the fair. "We strive to be a family friendly event and regret that Saturday's rodeo badly missed that mark," they said in a statement Sunday.
It wasn't clear if any action will be taken against the performers.
Perry Beam, who was among the spectators, said "everybody screamed" and "just went wild" as the announcer talked about having the bull run down the clown with the Obama mask.
"It was at that point I began to feel a sense of fear. It was that level of enthusiasm," Beam, a 48-year-old musician, said Sunday, referring to the reaction from the crowd that filled the fair's grandstand.
He said another clown ran up to the one wearing the Obama mask, pretended to tickle him and played with the lips on the mask. About 15 minutes into the performance, the masked clown had to leave after a bull got too close, Beam said.
Beam was at the rodeo with his wife and a student they were hosting from Taiwan. He said they were having a good time until the end of the rodeo.
"It was the usual until the very end at bull riding," he said. "As they were bringing the bulls into the chute and prepping them ... they bring out what looks like a dummy. The announcer says 'Here's our Obama dummy, or our dummy of Obama.
"They mentioned the president's name, I don't know, 100 times. It was sickening," Beam said. "It was feeling like some kind of Klan rally you'd see on TV," he said, referring to the Klu Klux Klan, which terrorized African-Americans for decades.
Officials with the Missouri Rodeo Cowboy Association, the organization that co-ordinated the rodeo, did not return phone calls seeking comment Sunday.
After Beam and his family returned home, he posted a photo of the clown in the Obama mask on his Facebook page. The photo and the posting were then promoted online by a blog, Showmeprogress.com, which elicited a huge response Sunday on Twitter.
Scott Holste, spokesman for Missouri's Democratic Gov. Jay Nixon, said Sunday in an email that Nixon "agrees that the performance was disrespectful and offensive, and does not reflect the values of Missourians or the State Fair."
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