HONG KONG — They are on the front lines of every demonstration, dressed in black T-shirts and pumping their fists as they march through Hong Kong’s sweltering streets. They organize on encrypted messaging groups and hand out helmets and goggles at rallies. When the police fired tear gas at them, they chased the smoke-emitting canisters and doused them with water.

Hong Kong’s youth are at the forefront of protests this month that have thrown the city into a political crisis, including a vast rally on Sunday that was perhaps the largest in its history. Organizers contend that close to two million of the territory’s seven million people participated, calling on the government to withdraw proposed legislation that would allow extraditions to mainland China.

For the many high school and university-age students who flooded the streets, the issue is much bigger than extradition alone. As they see it, they are fighting a “final battle” for some semblance of autonomy from the Chinese government.

“The extradition law is a danger to our lives,” said Zack Ho, 17, a high school student who helped organize a boycott of classes. “Once this passes, our rule of law would be damaged beyond repair.”