Anti-Iraq war activist Cindy Sheehan was arrested in the House of Representatives chamber shortly before President George W. Bush gave his State of the Union address because she refused to cover up an anti-war slogan on her shirt.
Ms Sheehan, who was attending the speech as the guest of US Democratic Congressman Lynn Woolsey of California, was taken from the Capitol in handcuffs and charged with unlawful conduct, said Capitol Police Sergeant Kimberly Schneider.
A photographer said Ms Sheehan entered the House gallery a few minutes before Mr Bush was to speak and was directed to her seat.
She had been seated for less than a minute when a plainclothes Capitol Police officer took her by the arm, said, "You've got to leave," and rushed her from the gallery.
Ms Sheehan did not resist and left with a smile.
Rather than hearing Mr Bush say in his speech that there would be no sudden US withdrawal from Iraq, Sheehan was being processed at the US Capitol Police headquarters near the Capitol.
Sergeant Schneider said Ms Sheehan was arrested because she was wearing a T-shirt with an anti-war slogan and refused to cover it up.
She said the unlawful conduct charge carries a maximum sentence of one year in jail.
Ms Sheehan, who became a central figure for the US anti-war movement after her son Casey was killed in the Iraq war, won wide attention with an anti-war vigil outside Bush's Texas ranch.
Sheehan and other activists were arrested in September for protesting outside the White House without a permit, a misdemeanor that carriers a $50 fine.