LONDON (Reuters) - British security forces are prepared for King Charles' coronation, security minister Tom Tugendhat said on Wednesday, hours after a man was arrested outside Buckingham Palace for throwing shotgun cartridges.

"We're in no way complacent" Tugendhat told Sky News. "I'm very glad to see that the police reacted incredibly quickly, incredibly professionally to the incident last night."

He added that months of planning went into preparing for the event, keeping in mind any number of different threats.

The incident outside the palace on Tuesday happened days before Saturday's crowning, which is expected to attract large crowds to London in addition to heads of state from around the world who are invited for the formal ceremony.

Tugendhat told Times Radio the event was "one of the most important security operations that the country has put into plan."

"The police are, to put it mildly, all over it, and our intelligence and other security forces are extremely aware of the challenge that we face," he said.

(Reporting by Sarah Young; writing by Muvija M, editing by Paul Sandle)