A man has been arrested on suspicion of urinating at the Westminster memorial dedicated to PC Keith Palmer.
The incident is believed to have taken place on Saturday afternoon.
PC Palmer, 48, was stabbed while on duty during the Westminster terror attack on 22 March 2017. He was one of five people murdered by Khalid Masood.
The Met Police said officers had arrested a man on suspicion of outraging public decency.
He is currently in custody in Essex after presenting himself at a police station.
Speaking on Saturday in response to a photo posted on social media, Commander Bas Javid said: "We are aware of a disgusting and abhorrent image circulating on social media of a man appearing to urinate on a memorial to PC Palmer.
"I feel for PC Palmer's family, friends and colleagues.
"We have immediately launched an investigation, and will gather all the evidence available to us and take appropriate action."
MP Tobias Ellwood, who gave first aid to PC Palmer as he lay dying after being stabbed in the grounds of Parliament, said the image of the urinating man was "abhorrent".
The remembrance stone was placed at the Carriage Gates at the Palace of Westminster, where PC Palmer was attacked.
The Police Memorial Trust said the stone was a reminder of PC Palmer's sacrifice and heroism.