The detained activists, who include seasoned dissidents and a prominent rights lawyer, had been demanding that senior Communist Party officials publicly disclose their personal wealth, according to lawyers and rights advocates.

The campaign, started late last year with a petition drive that garnered thousands of signatures, has tried to piggyback on a pledge by President Xi Jinping to clean up the endemic corruption he says poses an existential threat to the ruling Communist Party.

In widely quoted comments published in January, Mr. Xi promised to take down “tigers and flies” — a reference to high-ranking officials and middling bureaucrats — but the public clamor for mandatory asset disclosure has so far received a tepid response from Chinese leaders.

The campaign, called the New Citizens Movement by its organizers, has picked up steam in the five weeks since Mr. Xi consolidated power by adding the title of president to his other titles, Communist Party general secretary and chairman of the Central Military Commission.

The change in leadership, the first in a decade, has given liberals hope, however faint, that Mr. Xi will shake up the status quo by increasing government transparency.

On March 31, a small group of anticorruption activists unfurled banners at a plaza in central Beijing, a move that quickly drew the police. Three of those arrested are still in custody, and a fourth was released on bail for health reasons, his lawyer said Friday. All four are charged with illegal assembly, a crime that carries a potential five-year sentence.

Of those arrested last week, four are still being held, according to their lawyers, who said the police had raided the homes of at least two detainees and confiscated laptops, video cameras and other items.

The arrests have both infuriated and disappointed reformers and human rights advocates, who say the crackdown bodes ill for Mr. Xi’s widely trumpeted war on graft. “The party promised to publish officials’ assets 30 years ago, something it has yet to do,” said Xu Zhiyong, a lawyer and founder of the New Citizens Movement who is being held under house arrest. “Clearly the government is afraid of this demand.”

The men arrested last week include Zhao Changqing, a democracy advocate who has been jailed several times in the past; Ding Jiaxi, a human rights lawyer; and two activists, Sun Hanhui and Wang Yonghong. All four are being held at Beijing No. 3 Detention Center, lawyers for the men said.

According to Chinese Human Rights Defenders, a nongovernmental organization based in Washington, the whereabouts of two other activists, Qi Yueying and Li Wei, were unknown on Friday.

Liang Xiaojun, a lawyer who represents several of those detained, said prison officials would allow him to see only one of the detainees, claiming that the others were still being interrogated by the police. “I doubt this case will go through normal procedures,” he said. “Can you imagine a trial for a group of activists who demanded that government officials disclose their assets? I don’t see that trial happening.”

Analysts say the crackdown on dissent, coupled with newly announced media restrictions and the absence of any new anticorruption initiatives, are gnawing away at any hopes that Mr. Xi will embrace the rule of law and clean government.

“These arrests do nothing to dispel the widely held opinion that public office is in essence a way to accumulate illegal wealth,” said Nicholas Bequelin, a senior researcher at Human Rights Watch in Hong Kong. “If the party is serious about rooting out corruption, it needs to stop placing itself above the law.”