Zhou Shifeng is one of more than 130 lawyers detained or called for questioning by Chinese state-security this month, according to tallies by rights groups.
The lawyer provided legal aid to families of children poisoned by milk powder from a powerful dairy firm, and this year defended an 81-year-old writer detained for criticising the ruling Communist party.
Zhou has reportedly not been able to meet with a family-appointed defence attorney. But the official Xinhua news agency said late Saturday that he "admitted guilt," and pleaded for a "second chance."
Xinhua said that nine other lawyers connected to Zhou's law firm had been detained on criminal charges, along with several associates.
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The detained lawyers "gave interviews to foreign media, spreading opinions attacking the party and the government, slandering the legal system and other such negative views," Xinhua added.
Those detained include female lawyer Wang Yu, known for defending poverty-stricken victims of forced demolition, sexual assault, illegal detention and other abuses.
In an apparent effort to put pressure on Wang's family, her 16-year-old son has been questioned several times this week by police in the city of Tianjin, a family friend told AFP.
China's courts have a near-100 percent conviction rate. State-run media said last year that police using torture to extract confessions was "not rare" in the country.
The ruling party says it hopes to promote "rule of law," and a growing number of lawyers over the past decade attempted to expose official abuses using the courts.
But President Xi Jinping has stressed the party's ultimate authority over the legal system, and limits on activism have tightened.