Story highlights The government accuses Pu Zhiqiang of "picking quarrels and creating a disturbance"

He was detained in May after attending a seminar marking Tiananmen Square anniversary

Pu took part in demonstrations in 1989 in the square that ended in a bloody military crackdown

His work has put him at odds with the ruling Communist Party

Chinese authorities formally arrested prominent lawyer Pu Zhiqiang on Friday for "picking quarrels and creating a disturbance."

His other alleged crime was "illegally obtaining citizens' personal information," Beijing police said on their official microblog, adding that the investigation into Pu is still ongoing.

Pu, 49, was detained in early May after attending a low-key seminar in a private home to mark the 25th anniversary of the Tiananmen Square massacre. State-run Global Times newspaper said in an editorial at the time that he had crossed "a legal red line" by associating himself with a topic still considered taboo in China.

Pu took part in the student-led demonstrations in 1989 that ended in a bloody military crackdown on June 4 of that year. He later become one of the best-known lawyers in China for defending human rights in courts as well as in the media.

A fierce critic of China's once ubiquitous forced labor camps, Pu took on several high-profile clients who were victims of the "re-education through labor" system. His cases gained nationwide attention and support, pressuring the government to re-examine the controversial system and leading to its eventual abolition late last year.

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Although his work had often put him at odds with the ruling Communist Party, Pu dismissed the risks in an interview with CNN last summer.

"I think I'm fine," he said. "I'm a moderate, and the government has treated me well. I'm a veteran lawyer and haven't made mistakes in my career. I'm not radical, and I don't threaten the government."

Pu's arrest comes as the latest development in a new wave of government crackdowns on human rights advocates. Police put nearly 100 people in detention or under house arrest before this year's Tiananmen anniversary, said Chinese Human Rights Defenders, a Washington-based monitoring group.

When President Xi Jinping took office in 2013, some activists hoped he would preside over a system more tolerant of dissent and discussion. His government, however, is now widely seen as tightening the screws on the work of activists and intellectuals, including the sentencing of Xu Zhiyong, another well-known human rights lawyer, to four years in prison in January after he pushed for financial transparency for senior officials.