A Hong Kong court on Tuesday sentenced 45 former politicians and activists in a mass trial that has decimated the city’s once vibrant pro-democracy opposition and served as a warning that resistance to Beijing can be costly.

The landmark trial is the most forceful use of a national security law Beijing imposed on Hong Kong in 2020 in response to months of large protests against Chinese rule. The prosecution of the activists, the vanguard of Hong Kong’s opposition, has delivered what experts described as a knockout blow to hopes for democracy in the city.

Their offense, according to the authorities: holding or taking part in an unofficial primary election.

In one fell swoop in 2021, the authorities arrested Benny Tai, 60, a legal scholar and opposition strategist; Joshua Wong, 28, a prominent pro-democracy activist; and dozens of others, including veteran former lawmakers and younger politicians who called for self-determination for Hong Kong.

Mr. Tai was sentenced on Tuesday to 10 years in prison. Several opposition politicians and activists, including Au Nok Hin, Andrew Chiu and Ben Chung, were handed terms of around six and seven years each. Mr. Wong was given a sentence of about four years and eight months.

The trial made clear that any form of dissent or criticism, however moderate, carried significant risk, analysts said. “If you are being critical of the authorities both in Hong Kong and in China, then it’s open season,” Steve Tsang, a Hong Kong-born political scientist and director of the SOAS China Institute in London, said in an interview.