A man has been held by armed police at Buckingham Palace in central London, it has been reported.
The arrest took place moments before the Queen passed by in her car, reportedly at around 10.40am GMT.
The Metropolitan Police said the man arrested was carrying a knife, but add that the incident is not thought to be terror-related.
Footage apparently showing the arrest has been posted on social media, and can be seen here.
The suspect has been taken to a police station in central London.
Britain remains on high alert following Monday's terror attack in Manchester, which saw 22 people killed at a concert by the US popstar Ariana Grande.
Scotland Yard have said armed troops are to be deployed to guard key locations like Buckingham Palace and the Palace of Westminster, as well as various embassies across London.
This is part of a plan known as Operation Temperer, which was implemented after Prime Minister Theresa May raised Britain's threat level to critical following investigations into Monday's attack.
Raids took place across South Manchester yesterday, and three people have been arrested this morning in relation to the attack at the Manchester Arena.