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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Sunday will be largely dry with some bright spells of sunshine after any mist or fog lifts and clears.
British Bankers' Association head Anthony Browne warned that some banks could be moving abroad within weeks due to Brexit uncertainty.
Labour said its plan would mean grieving families would not have to pay for either a burial plot or cremation for a child aged under 16.