Brian Padden contributed to this report.
Protests broke out Friday in Hong Kong immediately after the city's government banned people from wearing masks at public demonstrations.
Protesters, including some wearing masks, fanned out across the city, leading to rallies, as well as violent clashes.
Carrie Lam, Hong Kong's chief executive, announced the ban earlier Friday, invoking a British colonial-era emergency powers act that was last used to quell riots in in 1967. Lam said the new regulation, to take effect Saturday, is a "prohibition of the face covering" and is intended to "target rioters or those that resort to violence."
Following the announcement, thousands of demonstrators gathered in the city's central business district and other areas, shouting "Hong Kong people, resist!''
Groups of angry protesters attacked pro-Chinese businesses, vandalized subway stops, and set street fires, causing police to respond with tear gas fire.
Police said an officer fired a live shot in self-defense after he was attacked by protesters in the northern Yuen Long district.
More rallies are expected over the weekend.
The new law threatens anyone wearing masks at protests with up to one year in prison. Hong Kong residents can still wear masks in the street, but under the new law, they must remove the masks if asked by police.
The face mask ban also has exemptions for those who have legitimate needs to wear a mask.
Face masks have become common during protests in Hong Kong, even at peaceful marches, as people fear retribution from government officials or that their identities could be shared with mainland China.
Many Hong Kong residents also wear face masks to protect against pollution or infection, such as the outbreak of the deadly Severe Acute Respiratory Syndrome (SARS) that struck the city in 2003.
For the last four months, the city has been engulfed in unrest as democracy advocates engaged in increasingly confrontational tactics to fight against what they see as China's efforts to restrict Hong Kong's autonomy and civil liberties.