China has arrested, jailed or detained thousands of rights lawyers and activists since President Xi Jinping launched a sweeping crackdown in the summer of 2015.
Judicial officials and police are investigating the cases and finances at seven law firms that often work on rights abuses, lawyers at the firms told the New York-based Human Rights Watch.
With unusually high-level officials performing scrutiny on site for days, the duration of investigations has exceeded that of previous regular checks on such firms, it added.
"While China's human rights lawyers are no strangers to official harassment, these sudden, invasive probes send an alarming message," Sophie Richardson, the group's Washington-based China director, said in a statement.
China's justice ministry did not immediately respond to a faxed request for comment from Reuters.
China has defended the new measures against critics by saying it is a country with the rule of law, and that those found guilty are criminals who damaged social stability and endangered national security.
"There has been an open attack on rights lawyers under Xi, and the probes should be considered in the context of the attack," said Maya Wang, a China researcher for HRW based in Hong Kong.
"It is meant to tell lawyers that this is not over, that they had better be on their best behaviour for the 19th Party Congress," Wang said, referring to a five-yearly leadership reshuffle of the ruling Communist Party set to begin on Oct. 18.
Political checks and requirements for law firms have also been tightened under Xi, making it harder for lawyers to take sensitive cases, activists say, especially as law firms can be held jointly accountable for individual violations.
Chinese law firms are required to explicitly support the party and to set up an internal party branch, while lawyers are banned from expressing opinions that endanger national security or reject China's political system.
In the run-up to the party congress, Xi has also urged public security officials to ensure stability and maintain the party's authority and unified leadership.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)