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.
More top news
FA Chairman Greg Dyke has told ITV News he thinks there is "more to come" in the corruption investigation against Fifa.
JD Wetherspoon has had to pay out thousands of pounds in damages after it emerged another of its pubs refused entry to travellers.
A cool, largely clear night across the UK.