More than 370 protesters have been arrested as police fired teargas, pepper spray and water cannon at thousands of people protesting against a national security law imposed by Beijing.

The extent of Beijing’s mandate was made clear as full details of the law were released late on Tuesday, giving authorities sweeping powers to crack down on dissent and allowing China new levels of control over the semi-autonomous territory.

On Wednesday, the 23rd anniversary of the handover from Britain to China and the first day under the new law, Hong Kong’s chief executive, Carrie Lam, and government dignitaries from the city and Beijing drank champagne in celebration. Lam called th law “the most important development in relations” between Hong Kong and China since the 1997 handover.

By the afternoon, thousands of people had taken to the streets of Causeway Bay and Wanchai in defiance of protest bans. Police were seen pinning protesters to the ground, shooting pepper balls at people who heckled them, and targeting journalists with water cannon and rounds of pepper spray.

Police said 10 of the arrests were for offences related to the new security law, including holding signs or flags advocating for  Hong Kong independence. Of those arrested, one was a 15-year-old girl who was waving a Hong Kong independence flag.