A portrait of the Queen in Westminster Abbey has been defaced with spray paint, an abbey spokesman has said.
A 41-year-old man has been arrested on suspicion of criminal damage after being detained by security guards at the abbey, Scotland Yard said.
Campaign group Fathers 4 Justice said it thought a member was responsible but the protest was not official.
The oil on canvas by Ralph Heimans went on display in Chapter House in May and depicts the Queen in state dress.
It was completed last year for the Queen's Diamond Jubilee celebrations.
The incident comes nine days after the monarch and other members of the royal family attended a service at the abbey marking the 60th anniversary of her Coronation.
A spokesman for the abbey said: "In an incident at lunchtime today, a visitor to the abbey sprayed paint on the Ralph Heimans portrait of the Queen presently on display in the Chapter House.
"Until work can be done to remedy the damage it will, very regrettably, not be possible to have the painting on public view."
Buckingham Palace said it would not making any comment on the incident.
