Chinese security police have held an American graduate student since Friday on suspicion of participating in the theft of "state secrets," the U.S. Embassy confirmed today.

Lisa Wichser, 29, who has been teaching at a Peking school for about 18 months while compiling data for a thesis at the University of Denver, was arrested at 1 a.m. Friday in the Friendship Guesthouse, where she had been living, according to other foreign residents. Detentions of foreigners here have been virtually unknown in recent years.

Neighbors said she was taken away in handcuffs after several plainclothes police searched her quarters. The police reportedly told Chinese attendants that Wichser is a spy working for the CIA who had been under surveillance for two months.

According to several of her colleagues, Wichser began applying to Chinese authorities about two months ago to marry a Chinese economics student whom she met in the course of her work. The Chinese man is said to have been involved in sensitive research.

China's Foreign Ministry issued a statement today saying Wichser had "violated Chinese laws by engaging in activities that are incompatible with her status as a foreign teacher in China . She has been detained by our public security organ."

Chinese authorities have not filed formal charges against Wichser, who reportedly is being kept in one of Peking's detention centers. According to China's criminal code, a suspect can be held 10 days before charges must be lodged.

The U.S. Embassy, which was barred from sending a representative to see Wichser until Monday, has protested to the Chinese government about its handling of the case. A spokesman said the embassy will take "vigorous action . . . to assure that Wichser receives fair treatment and protection of her rights under Chinese law."

The spokesman said the embassy has not been apprised of the precise allegations against Wichser, although "we understand it relates to the alleged theft of state secrets."

"We are aware this has a broader interpretation in Chinese society than there would be in the United States," he said.

China's communist regime operates in almost total secrecy, revealing information only through carefully managed propaganda organs except for the most innocuous material. Everything from government reports to international news accounts is classified "for reference," restricting distribution to officials of high rank.

Secrecy is defined in the broadest of terms, and the party holds out harsh penalties for misusing state secrets to intimidate officials from dealing openly with foreign journalists, diplomats and scholars.

After a Chinese editor was sentenced to five years in prison last month for leaking details of a Communist Party meeting to a Japanese reporter, the official Peking Review offered this definition of "state secrets":

"Before they are made public, all of the party's private activities are state secrets."

Foreign correspondents and social scientists based in Peking have found it increasingly difficult in recent months to carry out their normal activities of collecting information in this chilled environment.

Except for official warnings and travel limits, however, foreigners generally have been able to work without physical restriction. Although Chinese who regularly see foreigners on unofficial business often get detained for questioning, Wichser is only the second foreigner known to have been held by the ubiquitous security police since the death of Mao Tse-tung in 1976. The first was a Soviet citizen convicted of spying in 1980 and sentenced to seven years in prison.

Wichser, who is said to be fluent in Chinese, is one of the growing number of Americans to come to China since the normalization of diplomatic relations in 1979 to work for the Chinese government as copy readers, hospital workers, English teachers or translators.

Wichser, according to her colleagues, came to China in 1980 in the hope of researching her doctoral thesis on the rural economy. She taught English last year to fulfill her work obligation and began teaching an economics course to Chinese college students this year.

Lisa Wichser's father, Herman, said in Indianapolis that "she's had an interest in Asia" since being an exchange student in Malaysia during high school. She attended Earlham College in Richmond, Ind., focusing on Chinese studies, and spent 10 months in Taiwan before going to graduate school at the University of Denver.

As a foreign expert in Peking, she had access to certain classified materials, said an American co-worker, adding that it is doubtful she would have come across anything sensitive enough to warrant her arrest.

Another American friend said Wichser frequently complained that she had difficulty getting simple research material from the library because she was officially designated a teacher, not a scholar. "It makes me wonder what it takes to have that happen to you," said the friend. "Out here, you feel kind of protected, and then somebody gets grabbed in the middle of the night and it scares you."

Through interviews with Wichser's neighbors and other informed sources, the following description of her detainment emerges:

About 1 a.m., Friday, several plainclothes police entered Wichser's building and told one of the Chinese attendants to tell her she had an urgent telegram awaiting her on the first floor.

When she walked down from her third-floor quarters, police shackled her with handcuffs and held her in the hallway while others went through her rooms and emerged with several papers.

Wichser was taken away without a struggle. Friends who inquired about the incident the following day were told by normally taciturn attendants that undercover police had records showing Wichser worked for the CIA after her graduation from college.

The U.S. Embassy spokesman denied that Wichser has any CIA or other government connection, saying she is a private student previously unknown to American officials in Peking.

He said U.S. officials who have visited her at the detention center found her "in good condition given the nature of the situation." Consular agents were able to give her some personal items, such as stationery, instant soup and reading material.

U.S. Senate majority leader Howard Baker, who arrived in Peking Sunday for an official visit, has had several meetings with Chinese leaders but was unavailable for comment on the incident.