EIJING -- Two of China's most prominent dissidents were suddenly arrested for criminal offenses on Monday evening in an apparent crackdown on their efforts to form China's first opposition political party.

Over the past six months, Xu Wenli, of Beijing, and Qin Yongmin, from the southern city of Wuhan, had been involved in a small and loose nationwide network of political activists who have become increasingly bold in their efforts to start what they call the China Democratic Party.

The two are longtime dissidents, each of whom has spent over a decade in jail for pro-democracy views and is accustomed to police surveillance, occasional house searches and trips to the public security bureau for questioning.

But relatives and human rights groups both agreed that Monday's arrests were unusual and far more ominous. In Beijing 20 officers from the Public Security Bureau burst into Xu's home at 9 p.m. on Monday with a detention form. They arrested Xu and spent the next three hours searching his house, confiscating his computer, fax machine, telephone, address book and various other documents.

"This is the third time this year they've come here but from their attitude, I think it was much more serious this time," He Xintong, Xu's wife, said. "The other times they just took the fax. This time there were more officers and they were in more military uniforms. And they searched much more closely than they did before -- they even looked under the beds and in closets and they took phone receipts and bank transaction forms."

Ms. He is still waiting to be notified of the charges against Xu.

But Qin's family told a human rights group that Public Security officials notified them that he was being charged with "threatening state security," a grab bag of criminal charges that often carry sentences of three to 10 years. But sentences under those charges can be longer if the accused is considered a leader in serious anti-government activity.

Three other Democratic Party members were also arrested on Monday, said the Information Center for Human Rights and Democratic Movement in China.

Although the fledgling China Democratic Party was not specifically mentioned in any of the arrests, it was clearly the ultimate target. All of the men have been involved in repeated unsuccessful efforts to register the party with Chinese authorities in recent months.

And Ms. He said that when the police that ransacked Xu's home "found things related to the China Democratic Party, they went through them very carefully and then took them away."

Chinese leaders have not commented directly on the party. But just last week, Li Peng, the former prime minister and leader of the National People's Congress, pointedly told the German financial newspaper Handelsblatt that groups that "seek a multiparty system and try to negate the leadership of the Communist Party will not be allowed to exist."

The China Democratic Party has been a persistent source of irritation and anxiety for the Chinese leaders, an ever-buzzing mosquito whose bite is probably harmless but that is rarely silent and never seems to go away.

Its members are dissidents from more than a dozen Chinese cities. In one city or another they have had almost weekly run-ins with the authorities as they have tried time and again to gain some official recognition for the party, mostly by trying to register it with local officials. Party members have also tried to run in village elections.

It has carried out a small, but remarkably concerted and coordinated effort in a country that normally takes great pains to isolate its political dissidents.

None of the Democratic Party's efforts to gain recognition have been successful, and many have resulted in brief detentions by the local police and warnings that the party is illegal. But members like Xu and Qin have been unwilling to take no for an answer and when they hit one dead end, they back up and head out in another direction.

The Communist Party is effectively the only political party in China, and there had been no new parties allowed since the founding the People's Republic of China in 1949.

Some members are undertaking hunger strikes to protest the detention of party members in distant cities. In late October Xu theatrically announced that he was planning to set off on a drive from Beijing to eastern Shandong Province to show support for a party member there whose wife had been dismissed and whose phone and electricity had been turned off by local authorities.

On Nov. 8, long after they had been warned by local authorities that their pursuits were pointless and illegal, 53 Democratic Party members -- including Xu and Qin -- jointly applied to China's central State Council for permission to work on forming a new party, said the Information Center of Human Rights and Democratic Movement in China. The Information Center, a Hong Kong human rights group, sends out almost daily messages on the party's detentions and releases.

The party, whose by-laws support democratic ideals from election of officials to free speech, maintains that the Chinese Constitution does not specifically forbid alternate political parties. Its organizers add that since China signed the International Covenant on Political and Human Rights this fall, the government should let the activists have their say, as respectful opponents.

China's Criminal Code, under which Qin is charged, goes a long way to insure that new political parties never get off the ground. A chapter that is called "Threatening State Security" lists as crimes, "violent or nonviolent activities aimed at overthrowing government authorities," "activities designed to change the basic nature of the state," and "the overthrow of the socialist system."

It is unclear how such laws will be applied to the activists arrested this week. Indeed, Wang Youcai, a former student leader who was the first Democratic Party organizer to be detained for his work, was arrested in July on similar charges. Wang was released two months later to a relaxed sort of house arrest.

But on Nov. 2, police in his native Hangzhou took him away once again, for violating the terms of his release, said his wife, Hu Zhangxia, in a telephone interview. And, just Tuesday, as other Democratic party colleagues waited to learn of their fate, Ms. Hu said she was finally notified by the police that her husband had been formally arrested and was once again in the Public Security Detention Center.