HONG KONG (Reuters) - Dozens of Hong Kong pro-democracy activists will be sentenced later this week in a years-long subversion trial that has spotlighted what critics say are erosions to the city's freedoms under a China-imposed national security law.

The U.S. has described the trial and its guilty verdicts as "politically motivated," and has demanded that the defendants be released.

Dozens of prominent Hong Kong pro-democracy activists are scheduled to be sentenced Tuesday in the biggest case under a national security law that critics say has crushed political activism in the semi-autonomous Chinese city.

Police have stepped up security since the weekend, ahead of final hearing in biggest prosecution under Beijing-decreed national security law.