Two London police officers have been slightly injured detaining a man with a knife outside Buckingham Palace on Friday evening.

The Metropolitan Police force said the man, thought to be in his 20s, drove up to a police vehicle near the palace and officers spotted a large knife in his car.

It said the officers suffered minor arm injuries while arresting the man, who was being held on suspicion of grievous bodily harm and assaulting police.

The suspect was taken to a hospital for treatment of his own minor injuries, police said, and “no members of the public had contact with the man arrested.”

Police said it was too early to determine whether the incident may have been terrorism-related.

A large number of police vehicles could be seen in the Mall, the wide road outside the palace, and the area was cordoned off with police tape.

Witness Kiana Williamson said she saw officers trying to wrestle a man out of a car that had stopped near the palace. In less than a minute, “the man had been restrained and looked almost unconscious by the side of the road,” she said.

Buckingham Palace is the London home of Queen Elizabeth II and one of the city’s main tourist attractions.

The queen usually spends August in Scotland at her Balmoral estate with family members, however.

Police stepped up patrols around major U.K. tourist sites after two attacks with vehicles and knives this year on Westminster Bridge, which is near Parliament, and London Bridge.

Buckingham Palace, which is surrounded by tall gates, has seen past security breaches.

Last year a man convicted of murder climbed a wall and was detained on the palace grounds while the queen was at home.

In 1982 an intruder managed to sneak into the queen’s private chambers while she was in bed. Elizabeth spent 10 minutes chatting with him before calling for help.

A palace spokeswoman said the palace did not comment on security issues.