By Poornima Gupta

SAN FRANCISCO (Reuters) - A four-alarm fire roared from the roof of Pier 29, the future site of the America's Cup yacht race, along San Francisco's famed Embarcadero waterfront on Wednesday, but no injuries were reported.

More than 100 firefighters were dispatched to the scene of the blaze, which erupted shortly before 2 p.m. local time and sent flames shooting through the top of the vacant three-story terminal building, fire department spokeswoman Mindy Talmadge said.

The cause of the blaze was not immediately known, but Talmadge said the building had been under construction in preparation for hosting the 2013 America's Cup competition. She said the blaze did not appear to be of suspicious origins.

Within an hour, a major portion of the roof had collapsed and large chunks of the building's front archway had crumbled to the ground, as a plume of thick, gray smoke billowed into the clear blue skies over San Francisco Bay.

A crowd of onlookers and shoppers from nearby Fisherman's Wharf gathered to watch the dramatic blaze. Firefighters trained large streams of water on the building from water cannons mounted on several fire trucks to douse the flames.

The blaze was contained by 3:30 p.m., fire officials said.

A city transit official said trolley service along the Embarcadero had to be suspended because of emergency vehicle traffic in the vicinity of the tracks.

(Additional reporting by Emmett Berg; Writing by Steve Gorman; Editing by Cynthia Johnston)