The man, identified as Gu Hai'ou, from the northwestern city of Urumqi, threw a burning object at the portrait of communist China's first leader on Saturday afternoon, the Xinhua News Agency said. It said the lower left corner had a small burn mark, and authorities planned to replace the painting.
"Armed police are guarding the area and visitors are forbidden to enter the Forbidden City," Xinhua said.
Gu, 35 and unemployed, was detained and was being questioned by police, Xinhua said. It said he was treated last year in a mental hospital.
An officer who answered the phone at the Beijing police headquarters Sunday confirmed the report but refused to say whether Gu would face criminal charges or give other details. The man refused to give his name. Phone calls to the police station that oversees Tiananmen Square were not answered.
Portraits of Mao have hung on the Tiananmen Gate above the square since the 1950s.
In 1989, three men were detained for throwing red and black paint on the portrait during pro-democracy protests. They served more than 10 years in prison.