Police in China's northern Shaanxi province have arrested one of the nation's leading advocates of private property rights after officials posed as journalists and forged an e-mail from a prominent Hong Kong reporter to lure him out of hiding, friends and relatives said.

The arrest is a major setback for the plaintiffs in a landmark lawsuit against the government that analysts said has emerged as an important test of President Hu Jintao's willingness to promote rule of law and private property rights. The governing Communist Party has endorsed those principles as part of its drive to build a market economy.

A police official in Yulin, a city in Shaanxi, confirmed the arrest of Feng Bingxian, 60, a businessman who has led investors throughout the country in a civil suit that accuses the Shaanxi government of illegally seizing thousands of oil wells from them worth as much as $850 million. But the police official declined to discuss the methods used to apprehend Feng or the charges against him.

The Shaanxi case is one of the largest class-action lawsuits ever brought against the Chinese government. It involves some 60,000 investors who dug and developed oil wells in a 400-square-mile area around Yanan, the former revolutionary base of Mao Zedong, beginning in the mid-1990s. Local authorities allegedly approved the oil exploration, then confiscated the wells in 2003 after they began showing steady profits.

Feng's detention is the latest sign that Shaanxi officials have decided to maintain state control of the oil and quash the lawsuit. In May, police stunned the Chinese bar by arresting Zhu Jiuhu, a prominent Beijing attorney representing the investors, and charging him with disturbing social order through illegal assembly, apparently referring to meetings he held with his clients.

With Feng's arrest, police have also detained 13 of the 15 businessmen serving as lead plaintiffs in the case. The local authorities had made a priority of capturing Feng, a former party official from Inner Mongolia province who often traveled to Beijing to lobby for help and acted as an unofficial spokesman for the investors. At one point, officials offered a reward of nearly $40,000 for his arrest.

The local authorities' apparent decision to pose as journalists and tamper with e-mail to catch Feng suggests they were confident the party leadership supported their campaign to suppress the lawsuit as a threat to stability.

Feng's son, Feng Yanwei, said police notified him of his father's arrest Monday with a document indicating he was detained July 26 on suspicion of assembling to disturb social order. He said his father knew Yulin police were after him and had been in hiding for months, communicating with friends and relatives only by e-mail and instant messaging on the Internet.

Feng was last seen by the public in a special report on the case broadcast July 17 on Hong Kong-based Phoenix Satellite Television, which is widely available on the mainland. "There are times when police come around and I have to move from place to place," he said. "Things have gotten serious recently . . . I've become the main target they want to arrest now."

But he added: "I'm not afraid of being captured, just afraid of the consequences for our struggle if I am."

Soon after that appearance, friends and relatives said, Feng received an e-mail from a person who identified himself as Liu Bo and presented himself as an editor with China Central Television, or CCTV, the state broadcaster. Liu said he had seen the program and wanted to interview Feng, adding that his boss had already approved a show about the case, according to a copy of the e-mail exchange that Feng sent a friend before his arrest.

Feng asked how Liu planned to get around the ban on coverage of the case imposed by party propaganda authorities. Liu replied that his boss had not been informed of the ban and wanted to put together the program and broadcast it before censors could stop them.

Feng was wary, but excited, friends said, because a CCTV interview would represent a major breakthrough in the investors' efforts. In the e-mail exchange, he quizzed Liu about how he obtained his e-mail address and asked him for his phone number. Liu said his boss had given him the address, and ignored the request for his number. Liu instead pressed Feng to move quickly and meet him in the central Chinese city of Wuhan for a clandestine interview on July 26.

As late as the night of July 24, Feng had doubts. Using Yahoo's instant messaging program, he asked a friend in Beijing to visit the offices of the "Economy and Law" program on CCTV's Channel 2, where Liu claimed to work.

"If he really is a CCTV-2 person, then there should be no problem," Feng said, according to a transcript of the electronic conversation saved by the friend. "I need you to go see Liu Bo, and confirm there is such a person."

Feng warned his friend not to call CCTV because doing so might tip off the police. But he seemed optimistic, and speculated that Liu's boss might be "a bold and decisive newsman" with the guts to defy party censors. The next morning, Feng left a message telling his friend that she no longer needed to visit CCTV. Zeng Zimo, the Phoenix reporter who interviewed him, Feng said, had sent him an e-mail assuring him that Liu was legitimate. Feng also said he was on his way to Wuhan.

It was only after his arrest that Feng's friends discovered that there was no one named Liu Bo employed at CCTV-2.

In addition, Zeng said she never sent Feng an e-mail vouching for Liu. On the contrary, she said, she told Feng in an e-mail on July 26 that she did not know Liu and that he should be careful.

If Feng received a different e-mail from her, someone must have tampered with her Yahoo account or forged the message, she said. "What they did is illegal and despicable," she said. A spokesman for China Central Television denied that the station was involved in Feng's arrest but acknowledged that the e-mail account used by Liu belonged to a system used by CCTV two years ago. He said the station was investigating and declined to comment further.

The incident has prompted complaints among journalists at the station, who were already upset by a series of edicts from the party's propaganda department in recent months that restricted critical coverage of the government. "Now they're using us to suppress people too," said one reporter.

Days after Feng's arrest, the oil investors suffered another setback. The Shaanxi government reneged on a carefully negotiated deal to release their lawyer, Zhu, according to Gao Zhisheng, Zhu's Beijing attorney.

Gao said senior Yulin officials, including the police official in charge of the case, promised in a meeting with him on July 21 to release Zhu before Aug. 3 if he agreed to drop the investors' case. But when the deadline arrived, Gao said, the officials told him that a rival police faction -- the one that captured Feng -- had seized power and refused to honor the deal.

"The result of their power struggle is clear," Gao said. "They've cleaned out all the officials who are more reasonable."