Officers were called to Westminster Abbey around 12.30pm today after security guards detained a 41-year-old man.
He was arrested on suspicion of criminal damage and taken to a central London police station where he is in custody, a Metropolitan Police spokesman said.
The portrait, a Ralph Heimans' oil on canvas designed to commemorate the Queen's Diamond Jubilee, had been on display in the Chapter House for a few weeks before it was vandalised with paint.
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The family of the police officer who was murdered in the Westminster attack have thanked those who were with him 'in his last moments'.
Thousands of fans gathered for a public memorial honoring famous mother and daughter who died just a day apart.
More sunshine this Sunday