A first aid volunteer receives help after being tear gassed by riot police during a protest against police violence during previous marches, near China's Liaison Office, Hong Kong, China July 28, 2019.

A police officer detains a demonstrator during a protest against police violence during previous marches, near China's Liaison Office, Hong Kong, China July 28, 2019.

A pro-democracy protester attends a protest against police violence during previous marches, near China's Liaison Office, Hong Kong, China July 28, 2019.

Demonstrators are seen at a Hong Kong MTR (Mass Transit Railway) station during a protest against police violence during previous marches,Hong Kong, China July 28, 2019.

Journalists work amid tear gas during a protest against police violence in Hong Kong, China July 28, 2019.

Police officers disperse demonstrators during a protest against police violence during previous marches, near China's Liaison Office, Hong Kong, China July 28, 2019.

Demonstrators clash with police during a protest against police violence during previous marches, near China's Liaison Office, Hong Kong, China July 28, 2019.

Police officers line up during a protest against police violence during previous marches, near China's Liaison Office, Hong Kong, China July 28, 2019.

Demonstrators form a barricade during a protest against police violence during previous marches, near China's Liaison Office, Hong Kong, China July 28, 2019.

A demonstrator runs away from the riot police during a protest against police violence during previous marches, near China's Liaison Office, Hong Kong, China July 28, 2019.

Demonstrators use umbrellas and makeshift shields to cover, as they face riot police during a protest against police violence in Hong Kong, China July 28, 2019.

Riot police detains a demonstrator during a protest against police violence in Hong Kong, China July 28, 2019.

Police officers fire tear gas during a protest against police violence during previous marches, near China's Liaison Office, Hong Kong, China July 28, 2019.

An injured protester is seen after a clash with police during a protest against police violence during previous marches, near China's Liaison Office, Hong Kong, China July 28, 2019.

Riot police clash with demonstrators during a protest against police violence in Hong Kong, China July 28, 2019.

A pro-democracy protester reacts after throwing a tear gas canister back to the police, during a protest against police violence, near China's Liaison Office, in Hong Kong, China July 28, 2019.

A pro-democracy protester throws back a tear gas canister at the police during a protest against police violence during previous marches, near China's Liaison Office, Hong Kong, China July 28, 2019.

Demonstrators clash with police during a protest against police violence during previous marches in central Hong Kong, China July 28, 2019.

Police officers line up during a protest against what the activists see as excessive police force against protesters during previous demonstrations, near China's Liaison Office, Hong Kong, China July 28, 2019.

Medical assistant helps a demonstrator during a protest against police violence during previous marches, near China's Liaison Office, Hong Kong, China July 28, 2019.

Demonstrators run with the burning cardboards placed on a cart during a protest against police violence during previous marches, near China's Liaison Office, Hong Kong, China July 28, 2019.

A riot police officer detains a demonstrator during a protest against police violence during previous marches, near China's Liaison Office, Hong Kong, China July 28, 2019.

Demonstrators form a barricade during a protest against police violence during previous marches, near China's Liaison Office, Hong Kong, China July 28, 2019.

Riot police fire tear gas at demonstrators during a protest against police violence in Hong Kong, China July 28, 2019.

Demonstrators hold makeshift shields as they face riot police during a protest against police violence in Hong Kong, China July 28, 2019. The banner reads 'Liberate Hong Kong, the revolution of our times'.

Demonstrators take cover under umbrellas as they face riot police during a protest against police violence in Hong Kong, China July 28, 2019.

Demonstrators take cover under umbrellas as they face riot police during a protest against police violence in Hong Kong, China July 28, 2019.

Police officers fire tear gas during a protest against police violence during previous marches, near China's Liaison Office, Hong Kong, China July 28, 2019.