Amnesty International said on Friday Chinese police detained or harassed 50 people in the first half of this year for peacefully expressing their views or exercising their right of freedom of association.
The London-based human rights group said the 50 included political dissidents, human-rights defenders, worker rights' activists and relatives or friends of political prisoners.Of the group, 15 are still detained or restricted, Amnesty said.
In many cases, they were detained after issuing open letters or petitions to authorities in "apparent attempts by police to intimidate or silence them at politically sensitive moments," it said.
Many were detained in the run-up to the annual session of parliament in March, it said.
Another wave of arrests took place in May ahead of the ninth anniversary of the army crackdown on student-led demonstrations for democracy in Beijing.
The cases of Wu Ruojie, a rock singer, and Li Yi, a businessman, were "particularly striking and unprecedented in China's recent history of repression," Amnesty said.
The two men were sentenced without trial in April to three years of re-education through labor - an administrative punishment imposed by authorities outside the judicial process and usually involving detention in a forced labor camp.
Neither of the two are known to have been involved in dissident activities, Amnesty said. They were sent to a labor camp for allegedly divulging state secrets - revealing to foreign journalists the arrest of four poets in southern Guizhou province.