“I plead guilty,” Zhou Shifeng was quoted as saying in a report by the official Xinhua news agency late Saturday.
Zhou’s Fengrui law firm in Beijing is known for taking on politically sensitive cases.
“That the firm has breached the law is beyond doubt. [We] have committed illegal behaviour and even criminal behaviour,” Zhou was quoted as saying.
Zhou reportedly said that he had “wanted to be famous” and now takes responsibility for the damage to social stability caused by the firm, according to Xinhua.
State broadcaster China Central Television had earlier aired alleged confessions from another lawyer and employees at the firm.
“These confessions seem to be crafted to fit the government’s pre-existing accusations against the lawyers,” said Amnesty International’s China researcher William Nee.
“The government seems intent on vilifying them and making them out to be the main villains in a conspiracy,” Nee said.
The wave of arrests began after Wang Yu, a lawyer at the firm that had defended prominent dissidents such as Uighur economist Ilham Tohti, went missing on June 9.
Dozens more lawyers and activists were subsequently arrested.
By June 10, Zhou and three employees of the law firm had gone missing.
As of Sunday, at least 233 human rights lawyers and activists from across China have been detained, summoned by police or have disappeared, according to the Hong Kong-based China Human Rights Lawyers Concern Group.
Some 213 people, from more than 20 cities and provinces, have since been released, the group said.
Fourteen are still in detention and could face criminal charges, while six others are missing, according to the group.
The Public Security Ministry accused the detained lawyers of being part of a “major criminal gang” who had “seriously disturbed order.”
Most of those detained and questioned were lawyers, while the others were activists or other law firm employees, according to Amnesty, which called it the largest crackdown on lawyers since 2011.
Lawyers in Hong Kong have expressed “deep concern” over the arrests. On Sunday, a group of prominent barristers launched an international signature campaign over the crackdown.
Rights groups say that prosecutors in China rely heavily on confessions rather than evidence
Televised confessions from high-profile suspects including business executives, journalists and activists, have become more common in the past two years.
China has a conviction rate of 99.9 per cent. In 2013, just 825 of an estimated 1.16 million defendants were acquitted, according to Human Rights Watch.