HONG KONG — The authorities in Beijing have formally arrested one of China’s best-known human rights advocates on charges of disturbing public order, his lawyer and a friend said on Friday. It was the latest escalation in a case that has prompted a protest in China and criticism from the United States government.

“For Xu Zhiyong, this means the prosecutors have approved the matter, that they believe there is sufficient evidence for the allegations to be further investigated,” Mr. Zhang said in a telephone interview. “But that’s not a decision to prosecute. That could take months.”

Mr. Zhang declined to give details of the allegations against Mr. Xu, who was under informal house arrest for more than three months before his detention. A Beijing businessman who has campaigned for Mr. Xu’s release, Wang Gongquan, also confirmed that Mr. Xu had been formally arrested.

“As you move each step down the process — from detention to arrest and so on — the probability of it ending in conviction goes up dramatically,” said Joshua Rosenzweig, a researcher on human rights in China at the Chinese University of Hong Kong. “The chances are that Xu will face trial, and that makes it highly probable that he’ll be convicted.”

Mr. Xu, 40, rose to prominence in the broad “rights defense” movement of Chinese lawyers and advocates who have tried to use courts, media and the Internet for their cause. In recent years, Mr. Xu and others have turned increasingly to grass-roots campaigns. He supported a “New Citizens’ Movement,” which has urged the government to make public information about officials’ wealth. Many of Mr. Xu’s friends believe that he was arrested because of that campaign.

Under the new Communist Party leader, Xi Jinping, the Chinese government has continued to arrest and detain dissidents and human rights advocates seen as threatening party control. China Human Rights Defenders, an international advocacy group, has estimated that since March more than 50 Chinese involved in peaceful political activism have been detained or have disappeared, apparently into state custody.

Mr. Xu’s supporters have organized a petition drive to demand his release. Uzra Zeya, the American acting assistant secretary of state for democracy, human rights and labor, said in Beijing early this month that Mr. Xu was among the people whose detention she had raised in talks with Chinese officials.

This month, the police in southern China detained Yang Maodong, a well-known campaigner for human rights. Mr. Yang, who is best known by his pen name, Guo Feixiong, was also held on charges of assembling a crowd to disrupt order in a public place. If the two men were convicted of that charge, they could receive sentences of up to five years in prison.