HONG KONG — A Hong Kong court sentenced a protester to nine years in prison on Friday for terrorism and inciting secession, highlighting the power of a sweeping new national security law to deter those who might speak out against the authorities.

The protester, Tong Ying-kit, had faced up to life in prison after being convicted earlier this week. The case against Mr. Tong, who crashed a motorcycle into police officers while flying a protest flag, was the first brought under the security law, which was imposed on Hong Kong by China’s central government last year.

His case has heightened concerns among activists and legal experts that the security law is transforming Hong Kong’s judicial system, which is separate from mainland China’s. They fear that cherished civil and political rights are being trampled under a push to eliminate the sort of unrest and widespread opposition that was seen in the city during months of mass protests in 2019.

The power to interpret the security law rests with Beijing, and some observers say the outcome of Mr. Tong’s trial shows how much less space Hong Kong’s courts will have to weigh individual rights when considering security-related charges.