The boxing match billed as the "Fight of the Century" is over but the battle over smartphone video streaming of the Las Vegas showdown is just beginning.

Revelations that dozens of smartphone users streamed the Manny Pacquiao-Floyd Mayweather weekend fight have raised questions about how new technologies can get around copyright restrictions imposed at live venues such as sporting events and concerts.

There are no reliable figures, but thousands of people may have watched the fight via the Twitter-owned app Periscope, which allows any smartphone user to "broadcast" live video, and independent app Meerkat.

These viewers avoided the $100 fee for official pay-per-view video - as long as they did not mind reduced picture quality.

Twitter chief executive Dick Costolo appeared to boast about the feat when he tweeted at the end of the match, "And the winner is ... @periscopeco."

It highlights the conundrum copyright holders could face if they lose control of their rights to fans with smartphone apps.

Similar questions arose last month when HBO sent notices to Periscope over piracy of its Game of Thrones series. And these issues are expected to multiply as smartphone streaming gains popularity.

Under US copyright law, tech firms have a responsibility to remove copyrighted content "expeditiously" when notified of a violation. But at a live event, it is not clear how fast is fast enough.

"The law really does not have the tools for copyright owners to go after platforms that have content that is live streamed," said Bradley Shear, a Washington-area attorney specializing in social media and copyright issues.

The debate over these streaming apps "is going to restart the conversation about copyright protection in the digital age," Shear said.

Periscope and Meerkat say in their terms of service that they do not allow streaming of copyrighted content. But because there is no clear definition of "expeditious" the law may have been rendered useless by technology, according to Shear.

"This demonstrates the need to revisit our copyright law," he said.

Eric Goldman, co-director of the High-Tech Law Institute at Santa Clara University, agreed that when Congress wrote the Digital Millennium Copyright Act in 1998, "it did not contemplate live streaming".