The victim, in his 30s, was on holiday in London when he was chased by the group along Park Lane in Westminster.
CCTV footage shows the man running away as around eight suspects on four mopeds surround him.
Three of the mopeds then mount the pavement as they chase after him, knocking him to the ground.
The man was able to crawl away as his attackers rode off empty-handed, but suffered a broken leg, Scotland Yard said.
Met Police is appealing for information about the attack, which took place at 3.10am on 30 April.
Detective Constable Richard Kidd said: "This happened just after 3am when a group of four or possibly more mopeds riding together would have been conspicuous.
"I want to hear from anyone who recalls seeing such a group or who may have seen the mopeds being abandoned - or perhaps being stored in a lock-up or similar location - in the early hours of April 30.
"One of the mopeds would have sustained damage to its left side."
:: Anyone with information should contact police on 101 or Crimestoppers on 0800 555 111.