Stockholm — The detention of a Latvian-American tour operator by Soviet customs officials last week in Tallinn, Estonia, points to increasing Soviet nervousness about so called ''ethnic tourism'' to the Baltic republics.
Latvia is visited every year by some 3,000 tourists of Latvian descent, according to Soviet statistics. This summer, a large number of tourists planning to visit Latvia from overseas will also attend a Latvian song and cultural festival in Munster, West Germany. The latest incident may be a warning that nationalist sentiments from the festival shouldn't be imported to Latvia.
The tour operator, Biruta Sereda, of San Mateo, California, was briefly held in Tallinn. She was prevented from leaving the Soviet Union with group of more than 40 Latvian-American tourists while returning from Latvia via the Estonian capital. Mrs. Sereda plans to have several groups going to Latvia before the festival.
Her group returned unaccompanied by ferry to Helsinki, where several tourists alerted US diplomats and news agencies.
Mrs. Sereda was released two days later. She told business associates in Helsinki that the reason she had missed returning on schedule was the time required by customs officials to catalogue gifts and jewelery she was taking out of Latvia.
Members of the tour group also reported that Mrs. Sereda was ill and fainted during what they said was a very intensive customs search.
Sources familiar with the incident, however, suggested that the California-based tour operator was apparently ''set up'' by the Latvian KGB in a case typical of the way the Soviet police and intelligence service ''wages psychologial warfare'' against tour operators and tourists who speak Baltic languages.
At the same time, frequent travelers to Latvia said the thorough customs search reflected an unprecedented worry by Soviet authorities about items tourists may be taking out.
One Western diplomat, recently in Leningrad, also spoke of intensive border searches, suggesting the Soviets are more wary of all travelers, not only emigre tourists.
The informants said that once the tour operator had been isolated from her group and frightened and exhausted by the ''brutal'' customs search, an anonymous young Latvian official showed up, and professing to ''save'' her from the Tallinn customs officials, started to politely ask her whether any foreign secret services were paying her to operate tours to Latvia.
According to tourists who have undergone similar ''discussions,'' the Latvian official most likely referred to details of the tour operator's activities in Latvia that could only have been obtained by clandestine surveillance and cautioned that her business could be terminated by offending the authorities. The official, in effect, implied he was the tour operator's KGB ''case officer, '' but never disclosed his name or occupation.
For a number of tour operators such as Mrs. Sereda, for whom trips to Latvia comprise their main business activity, such a veiled threat ''amounts to blackmail,'' according to one source.
Because of economic interests and personal ties with Latvia that are often involved, incidents with tour operators are usually downplayed, and the KGB activities can be repeated against other persons since they receive virtually no publicity.
Sources familiar with such incidents also said the US and other Western diplomats tend to categorize such cases simply as rough treatment by Soviet authorities rather than see them as part of a deliberate campaign in an esoteric conflict between the KBG and emigres.
When questioning tour operators, Latvian KBG operatives often pose as officials of a ''visa department'' located on Henri Barbiss St. in Riga. They sometimes invite tourists for coffee and cakes, claiming to be checking up on tourists' welfare in tourist services, but officials of the same department detained and harshly interrogated a Latvian mother and daughter from Sweden in early 1983.
Sources in Mrs. Sereda's tour group said the operator may already have been called for polite questioning while in Riga. The authorities were apparently worried about the large number of younger tourists with the group and may have inquired whether any of them held anit-Soviet views.