Constable's masterpiece The Hay Wain has been attacked by a protester in London's National Gallery.
A man is in custody at a London police station after being arrested at the Trafalgar Square gallery shortly after 1pm on Friday, 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.
Fathers4Justice named the man arrested as Paul Manning, who claims he recently lost a legal battle involving his son, and said he had carried out "a final act of desperation".
In a statement posted on its website he wrote that his heart, conscience and love "pushes me on to some sort of future action, some sort of path that will get me back to my dear son or it may not?"
He added: "It will be an action that may lead to my incarceration possibly even to my own death.
"For I tell you this and take note: my own nation or government or some judge who knows little of my deep love for my child is NOT going to take my son from me or prevent me from seeing him!"
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."
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 that from now on it was refusing to engage with government, police, courts, the judiciary and any other organisations involved in family law.