Police said the officers did not require hospital treatment. No other injuries were reported.
Police said it was too early to say whether the incident was terrorism-related.
A large number of police vehicles could be seen in the Mall, the wide road outside the palace, and the road was cordoned off with police tape.
Witness Kiana Williamson said she saw police 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 one of London's main tourist attractions, and the London home of Queen Elizabeth II.
The queen, however, usually spends August in Scotland at her Balmoral estate with family members.
Police stepped up patrols around major U.K. tourist sites two attacks with vehicles and knives earlier 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 while the queen was at home, and was detained in the grounds.
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.
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