A man was left unconscious after being hit by a piece of concrete that fell from a building at a demolition site in central Tokyo on Monday morning, police said.

The man, a security guard who was directing traffic at the site in the capital's Minato Ward, was sent to hospital with his head bleeding, according to the Metropolitan Police Department.

The piece of concrete, apparently from an exterior wall, is believed to have fallen from around the fifth floor of the building through a construction enclosure.

The injured man is in his mid-60s.

An exterior wall suddenly came off, and something like a stone fell down, according to another security guard who was nearby at the time of the accident. Passers-by could have been injured in the accident, the guard said.

The demolition work started in April, according to a notice posted at the site. Construction of a 12-story office building is scheduled to start in January next year.