An American scholar who has been held by security police for six days on suspicion that she was involved in stealing Chinese "state secrets" will be released Thursday morning and given 48 hours to leave China, according to U.S. Embassy sources here.

Lisa Wichser, 29, a University of Denver graduate student who has been teaching in Peking for about 18 months while collecting material for a doctoral thesis, had been accused of violating Chinese laws "by engaging in activities that are incompatible with her status."

The U.S. Embassy protested China's handling of the case, claiming that American officials had not been notified of the arrest in a timely fashion and had been prevented from visiting Wichser for nearly four days despite a consular convention that provides for speedy access to imprisoned Americans.

"In a case like this, all a consular agent can do is insist on procedural justice," said a senior U.S. diplomat. "It's not for us to interpret Chinese law."

U.S. Senate Majority Leader Howard Baker (R-Tenn.), who has been in Peking on an official visit, had pressed Chinese leaders to permit regular contacts with Wichser, only the second known foreigner to be detained here since the Cultural Revolution ended in 1976. The first was a Soviet citizen convicted of spying in 1980.

Although Chinese authorities never pressed formal charges against Wichser, they told embassy officials that she was being held in an investigation into "the alleged theft of state secrets," according to a U.S. spokesman.

Chinese attendants at the guest house where Wichser had been arrested Friday said they had been told by undercover police that the American woman was a spy working for the Central Intelligence Agency who had been under surveillance for two months.

According to Wichser's colleagues, she had applied to Chinese authorities about two months ago to marry a Chinese economist. He is said to have disappeared in recent weeks and is believed by foreign sources to have been arrested.

Foreign diplomatic sources believe Wichser was detained to gather evidence against the children of high-ranking Communist officials who have leaked classified material to foreigners.

"They're trying to squeeze Wichser to get at the cadre children," said an Asian diplomat with good Chinese contacts.

Secrecy is broadly defined in China, and a "state secret" can be almost any government report or news analysis that has not been cleared for public circulation through the official propaganda channels.

"The Chinese are showing bounding sensitivity to the sharing of what they regard to be privileged information with foreigners," said a Western diplomat. "The Wichser case is just one manifestation of it. It's clear they're getting into a very strict interpretation of their loose laws on what constitutes a state secret."

China never intended to prosecute Wichser, said the diplomat, but hopes to use her as an example to intimidate other foreign scholars and journalists based in China.

Wichser, who has been held in a Peking jail, has been allowed two visits by U.S. officials since her arrest. The embassy, notified of her detention 36 hours later, was prevented from sending representatives until nearly four days later.

The consular convention signed by China and the United States last year requires that the embassy be notified within four days of the arrest of a U.S. citizen in China and be allowed to send official visitors within two days of notification.

A U.S. consular officer who was permitted to vist Wichser for 30 minutes this morning found her to be "in relatively good spirits," according to an embassy spokesman. She said that she "had been treated considerately," the spokesman said.

Baker, who said he had been given assurances from Vice Minister Zhang Wenjin that China would live up to the consular convention, gave no indication at the afternoon meeting with reporters in Peking that the Chinese had agreed to release Wichser.

Baker said he does not think Sino-American relations, which already are strained over U.S. arms sales to Taiwan, will be further bruised by the Wichser case as long as Peking adheres to the convention.

He indicated that his three days of talks with Chinese leaders were dominated by the Taiwan arms sale controversy.

Baker said talks with Vice Chairman Deng Xiaoping and other Chinese leaders centered on the Taiwan Relations Act, which was passed by Congress in 1979 to reassure Taiwan that its security needs would be met after Washington shifted its recognition earlier that year from Taipei to Peking.

Baker said he told Deng that he opposes any move to remove provisions from the act requiring U.S. help in assuring Taiwan's defense.