John Constable's masterpiece "The Hay Wain" was attacked in Britain's National Gallery on Friday by a protester believed to be linked to the campaign group Fathers4Justice.
A man was arrested at the prestigious gallery after gluing a 10-centimetre (four-inch) photograph of a young boy to the 1821 landscape.
The National Gallery said no lasting damage had been done to the painting, which is one of Britain's best-known works of art.
Fathers4Justice released a statement from a man named as Paul Manning who said a custody battle with his former partner had forced him to take "drastic action".
The British group, which campaigns for fathers' fair access to their children following separation from the mother, also released photographs of Manning holding an image of his son, with the word HELP scrawled on it.
A second photo showed the image glued to the famous canvas.
The attack came as a Fathers4Justice campaigner appeared in court accused of vandalising a portrait of Queen Elizabeth II in London's Westminster Abbey two weeks ago.
Fathers4Justice had called for dads to take "independent weekly direct action", saying it was abandoning a five-year "attempted engagement with the political establishment".
London's Metropolitan Police said a 57-year-old man was in police custody after being arrested shortly after 1 pm (1200 GMT).
A spokeswoman for the National Gallery said the painting was already back on display.
"Conservation staff were on the scene very rapidly and the painting was removed for treatment," she said.
"No damage to Constable's original paint occurred and there is no lasting damage to the painting."
Set up in 2001, Fathers4Justice have gained a reputation over the years for headline-grabbing stunts.
Their activists have scaled buildings such as Buckingham Palace dressed as superheroes, and in 2004 they sparked a major security alert at the British parliament when they pelted then-prime minister Tony Blair with flour as he was speaking.
A spokeswoman for Fathers4Justice said the group was now encouraging fathers to write "help" or place 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," she said.