Her actions come amid heightened security around George Bush, whose advisers fear an al-Qaida attack. His entourage has insisted on top-level security, ruling out traditional events such as a procession in a horse-drawn carriage with the Queen.
Veteran Quaker anti-nuclear protester Lindis Percy scaled the 6m iron gates in front of the palace where Bush will stay for three nights after he arrives late on Tuesday.
After unfurling the American flag upside-down, with the inscription "Elizabeth Windsor and Co. He's not welcome" on it, Percy clung to the gate for more than two hours before climbing down and being taken away in a police van.
But her bold stunt will not please the authorities, who have cancelled all police leave in London. A record 14,000 officers will be on duty over the three-day state visit in anticipation of large-scale protests over the Iraq war.
Organisers of a major protest march through London, planned for Thursday, said they were pleased after police finally granted them permission to parade past key government buildings where Bush will be holding meetings.
Thousands are gearing up to
demonstrate against the visit
An estimated 100,000 people will take part in the demo.
At the climax of the march in Trafalgar Square, protesters hope to topple a giant effigy of Bush in a symbolic repeat of the destruction by US troops in April of a statue of Iraqi President Saddam Hussein.
"This does not mean that we support Saddam. He was a nightmare," says Mell Harrison, 32, of the Theatre of War activist group said.
"But what Bush has done has not helped ordinary Iraqis at all. There is chaos there. Children are dying every day."
Police snipers will line the president's route on rooftops, while the capital's rapid-response armed units will be on full alert.
However, an ICM poll published in a national British newspaper said that 43% of British voters welcomed Bush's visit, while 36% opposed it.