Cameron was seen in video images backing away from a man with long dreadlocks running towards him and then being rushed to his car as the man was hustled away by security officers.
The police said in a statement that the 28-year-old was arrested by officers but later released without charge, following the incident in Leeds in northern England.
The unit responsible for the prime minister’s protection “will now conduct a review of the incident,” it said.
Cameron played down the security scare by thanking his bodyguards and making a joking reference to former deputy prime minister John Prescott, who punched a protester in 2001.
“John Prescott was in the room as I gave the speech so as I left the room, I thought the moment of maximum danger had probably passed. But clearly that wasn’t the case,” he said.
Police said in a tweet there was “nothing sinister” and it was “just a man in the wrong place at the wrong time”.
They were quoted as saying that the man, who was wearing headphones, a tracksuit and sports shoes, may have been a runner jogging to a nearby gym.
But Conservative lawmaker Mark Pritchard said that the incident was “a clear breach of security and could have been far more sinister in outcome”.