The Chinese government, perceiving a grass-roots challenge to its authority from an outlawed spiritual sect, is fighting back with the same tool favored by political dissidents to evade state control: the Internet.

Members of the Falun Gong sect in the United States, Britain and Canada have reported recent assaults on their World Wide Web sites by a variety of electronic methods commonly used to block or penetrate sites operated by Chinese dissident groups abroad. The members accuse Chinese security officials of being behind the harassment.

The government also has launched an anti-Falun Gong Web site and filled it with articles assailing the exercise and meditation group as a grave threat to the mental and physical health of Chinese society.

Internet technology has been hailed as a powerful liberalizing force in China, giving the country's estimated 4.5 million Web users access to uncensored information about their homeland and the rest of the world that is otherwise unavailable to them. But the same cyberspace links are available to police officials around the nation, who view the Internet as an effective means of monitoring dissent.

The Chinese government intensified its online surveillance in May, when agents of the Ministry of State Security paid a visit to the offices of Internet service providers here and connected two new monitoring devices to the providers' computers, sources said.

They said the new equipment tracks individual e-mail accounts and thereby fortifies the Beijing government's long-standing effort to block what it describes as "harmful" Internet sites -- such as those run by overseas Chinese-language newspapers.

In past years, Chinese Web users have been questioned by security agents because they received VIP Reference, a pro-democracy e-mail bulletin that originates in the United States. Lin Hai, a computer entrepreneur in Shanghai, was arrested last year for supplying Chinese e-mail addresses to VIP Reference.

The government's July 22 order banning Falun Gong has further focused Chinese authorities on the power of the Internet. Officials noted that Li Hongzhi, the New York-based founder and leader of the movement, uses it to communicate with his followers in China, who are estimated to number more than 10 million. The government responded by using filtering software to block Chinese Internet users from visiting Falun Gong sites.

Falun Gong Web sites overseas have also been bombarded with electronic requests that block normal computer traffic. In addition, electronic intruders have attempted to gain access to the inner workings of the sites, according to Falun Gong members in the United States.

One U.S.-based Falun Gong Web site operator said that an analysis of a computer trail led him to believe that Chinese police were linked to at least one of the attempts, but he left open the possibility that a sophisticated hacker could have created such a trail. The anonymous nature of the Internet makes the operator's claim difficult to prove.

Inside China, the crackdown on Falun Gong has resulted in thousands of detentions and the destruction of millions of the group's manuals. The government appealed to the international police organization Interpol for help in apprehending Li, who Beijing alleges is responsible for the deaths of at least 700 followers. But the Paris-based investigative body refused today to provide assistance because of the political and religious overtones of the case.

As the government has become more fluent in its use of computer networking, it has also applied Internet technology to more conventional crime-fighting.

China's state-run press reported today that police around the country are engaged in a "war on the Web," a war in which the Internet has been used to identify and detain more than 15,000 suspected criminals since last month in three provinces -- Sichuan, Hebei and Guangdong. Among those arrested, the reports said, were people wanted for murder, smuggling, counterfeiting and theft.

Last month, teams of police technicians in Beijing and provincial capitals put detailed information about tens of thousands of alleged criminals into online databases that can be accessed easily by far-flung local police forces, according to the official New China News Agency.

In one Internet success story touted by the news agency, police in the central Chinese province of Sichuan closed a case involving a top executive of a state power company. The executive was wanted in the coastal province of Guangdong, where he was accused of stealing $33 million from the enterprise, but he was ultimately taken into custody in Sichuan's capital, Chengdu. The death penalty is often prescribed for perpetrators of major economic crimes in China.

Using the new online database, police in Sichuan have arrested 13,000 people in their province-wide manhunt, according to the New China News Agency. Of the 13,000, 1,400 were determined to be fugitives and detained. That figure included 58 described as major criminals who had been sought for such crimes as murder and large-scale corruption, the news agency said.