Constable's masterpiece The Hay Wain has been attacked by a protester in the National Gallery.
A man is in custody at a London police station after being arrested at the Trafalgar Square gallery shortly after 1pm, Scotland Yard said.
The protester, thought to be linked to Fathers4Justice, glued a four-inch photograph of a young boy to the 1821 painting in room 34 but did no lasting damage to the work of art, a gallery spokeswoman said.
It comes as Fathers4Justice said it was abandoning its five-year "attempted engagement with the political establishment" and called on fathers to take "independent weekly direct action" in the spirit of the Suffragettes 100 years ago.
The gallery spokeswoman said: "Conservation staff were on the scene very rapidly and the painting was removed for treatment.
"No damage to Constable's original paint occurred and there is no lasting damage to the painting."
She added that the picture was due to to back on display this evening.
The Hay Wain is one of the country's most recognisable works of art.
John Constable's oil painting shows an idyllic rural scene with a cart - the eponymous hay wain - in the river Stour in Suffolk.
The national gallery spokeswoman said it would investigate the security breach but praised "the prompt action and quick thinking" of staff who intervened before any more lasting damage could be done.
The attack on the painting came as a Fathers4Justice campaigner appeared in court accused of vandalising a portrait of the Queen in Westminster Abbey.
Tim Haries, 41, from Doncaster in South Yorkshire, appeared at Southwark Crown Court in London today charged with criminal damage to the Ralph Heimans canvas, which was daubed with spray paint on June 13.
He was bailed to appear at the court again in September.
Fathers4Justice said today that from now on it was refusing to engage with government, police, courts, the judiciary and any other organisations involved in family law.
Instead it said it wants fathers to take direct action in "an attempt to defend themselves and the 1,000 families a week destroyed in the secret family courts"
It also said it was considering closing down all its social media outlets and was refusing to deal with the national media because of its "deliberately inaccurate and misleading reporting of the campaign and the crisis in the family courts".
"We have reached a tipping point in the campaign," a spokeswoman said.
"For every 1,000 families destroyed each week in the family courts, fathers should respond in kind with peaceful non-violent direct action, by either writing 'help' or placing pictures of their children in significant places where they are visible to the world."
"We can no longer stem the tide of desperation and anger of fathers who have had their families destroyed and their hopes betrayed by a government that promised equal parenting but only delivered desperation."
"Fathers have a duty of care to protect themselves and their families and give themselves a voice which the politicians and media have denied them for over a decade."