Before her arrest more than a year ago, Liu Di was a university psychology student virtually unknown to all but a few Internet users who read her postings, in which she talked about everything from the human psyche to contemporary literature.

Now, the young woman has been jailed as a dissident for more than a year and has become the center of international attention as human rights activists push for her release. No charges have been filed against her.

Before her arrest, 23-year-old Liu Di lived with her grandmother, Liu Heng. Mrs. Liu is a retired journalist who herself was thrown in prison in 1957 after the government of Communist leader Mao Zedong labeled her an "incorrigible rightist." Speaking briefly before her telephone went dead, she said her granddaughter does not fit the mold of a radical student activist. "I'm still not sure about the whole situation. However, I think she made some mistakes," says Mrs. Liu. " What mistakes she made, I'm still not sure about. The government was suspicious and arrested her.

Using the alias of Stainless Steel Mouse, Ms. Liu posted essays on-line expressing sympathy for jailed Internet activists and criticizing a government decision last year to close thousands of Internet cafes.

Peng Dingding, an Internet activist who has taken up her cause, says Ms. Liu's postings challenged the communist system but were not enough to make her a prominent dissident. He describes one of her on-line comments. "She said, let's make an experiment. She said let's take the communist manifesto and take off the cover, and read it to the people, and see whether they agree with the manifesto," he said. Mr. Peng said what he finds most disturbing about Ms. Liu's case that authorities have yet to say why she is being detained.

Liu Di's detention has raised fear that anyone could be arrested, especially at a time when rising unemployment and other social problems are driving many young people to vent their concerns on the Internet.

The global human rights advocacy group Amnesty International says at least 40 people are currently jailed in China for Internet-related offenses. The number is small when compared with the 68 million Internet users in the country. Some legal experts say the small number of arrests could reflect the difficulty that authorities have in controlling something as large as the Internet.

The government has formed an Internet police force that is estimated to employ thousands of people to monitor cyberspace activity. However, observers say the job of tracking millions of Internet users is too large for any police force, regardless of how well staffed.

Authorities are apparently having trouble getting charges against Ms. Liu to stick. Family members last month said prosecutors bounced the case back to police, demanding that investigators come up with better evidence.

Still, Ms. Liu remains in prison.

Chinese law says criminal suspects may be held without charges for up to two months, with extensions possible in cases that are deemed severe. With few leads to go on and no large kingpins to target, the government is using those it detains as threads to go after others it knows are posting politically unfavorable material.

On October 28, plainclothes police raided the home of Du Daobin, a 40-year-old office worker in Hubei province who posted essays criticizing government corruption. Relatives say agents arrested Mr. Du after he had signed an on-line petition demanding the release of Liu Di.

"I don't know why they arrested my husband," said Huang Chunrong, who has been visiting jails, searching for her husband. "I feel he just wrote a few articles. He didn't do anything else. I don't think he did anything radical like they say he did. I don't understand what is being called 'radical.' He just wrote some articles. '

Michael Davis teaches law at the Chinese University of Hong Kong. He sees the random arrests as an indication that Chinese Internet patrols are not finding their work easy. "We have seen there's a sort of crackdown that is sporadic at best because I think they realize the difficulties of controlling the Internet," he says.

Professor Davis says the authorities' approach seems to be along the lines of an old Chinese proverb that says, "kill the chicken to scare the monkey." "One of the techniques we've seen recently is one of arresting someone prominently and visibly so that it becomes a warning to others to back off in whatever they may be doing on the Internet."

A reporter attempted several times to interview officials with China's Public Security Bureau in Beijing. Officials did not return calls. The case of Liu Di has triggered worldwide protests by human rights advocates to demand that China release those it is holding for Internet-related offenses.