A rooftop fire broke out in Tokyo's crowded Shibuya Ward on Wednesday evening, as revelers flocked to the Halloween mecca for a massive annual street party.

Firefighters were working to extinguish the blaze, which occurred at a building near a police box on the area's iconic Center-gai street, according to media reports and video posted to social media.

The Tokyo Fire Department said the fire, which broke out around 6 p.m., had spread from the sixth floor of a building complex to its roof, public broadcaster NHK reported.

There were no immediate reports of injuries, but the scene of the blaze was Udagawacho, near the famed "scramble crossing" in front of JR Shibuya Station, where thousands of partygoers were gathering for the Halloween celebrations.

Police were bracing for massive crowds to mark the holiday, with a possible repeat of behavior that saw a small truck overturned and five people arrested on the weekend likely at the front of their minds.

It was unclear how, if at all, the fire would affect the raucous affair, but several hundred members of the capital's police force were being mobilized in the Shibuya area to monitor rowdy crowds through early Thursday.

Special officers dubbed "DJ police," who aim to marshal crowds with a spirit of goodwill, were also dispatched to the area.

Between Saturday night and Sunday morning, five people were arrested in Shibuya on suspicion of groping women or trying to take upskirt photos or videos.

Shibuya Mayor Ken Hasebe has urged people to be on their best behavior and to observe the law, criticizing the weekend's incidents as "absolutely intolerable" in a statement released Monday.