The woman, wearing a straw hat, was escorted away from the palace gates after unfurling a hand-drawn sign, proclaiming: “Not My King” on Monday.
She was led away by five police officers and made to stand on the other side of the road.
A witness said she was not arrested and was allowed to continue her protest.
Photojournalist Joshua Bratt told The Standard: “They asked her to move to the opposite side of the road from the entrance as they were trying to keep the flow of people moving.
“In fairness to the police, and I’m not always their biggest fan, they walked her to the crossing and allowed her to carry on.
“She wasn’t arrested and stood with her sign with the media and other people waiting for the King.”
He added: “She did say that it’s not a democratic thing for a King to be head of state without it being put to the public.”
A Scotland Yard spokesman said: “A member of the public was asked to move away from the Carriage Gates outside the Palace of Westminster this morning in order to facilitate vehicle access and egress through the gates.
“She was not arrested and was not asked to leave the wider area.”
It came after another woman was charged after holding an ‘abolish monarchy’ sign during the Accession Proclamation for the new King in Edinburgh.
Moments before the proclamation of Charles III as new king on Sunday afternoon, a demonstrator appeared in the crowd opposite the Mercat Cross.
She held a sign saying ‘f*** imperialism, abolish monarchy’.
Officers appeared behind her and took her away, prompting the crowd to applaud.
Another man heckled Prince Andrew shouting “Andrew, you’re a sick old man,” as he followed the hearse carrying the Queen’s casket as it moved down the Royal Mile in Edinburgh.
His comments were picked up on broadcast microphones before other outraged mourners pushed the protester into the hands of the police who dragged him away from the metal barriers.
The Duke of York will be back in military uniform for the final vigil around the Queen’s coffin during the lying in state, but not for the state funeral or other ceremonial elements in the coming days.
Andrew, the late monarch’s disgraced second son, stepped down from public life after the furore over his friendship with paedophile billionaire Jeffrey Epstein, and paid millions to settle a civil sexual assault case to a woman he claimed never to have met.