The boulevards in this city on the Neva River are alive with American fur buyers, athletes and tourists, trumpeting an expectation -- voiced by Soviet officials and western diplomats alike -- that 1986 will bring the biggest influx of visitors from the United States since the heyday of detente.

But officials on both sides interpret the anticipated surge in American visitors as far more significant than just a boost in tourism. They say it represents a mending of the 1984 rift between Washington and Leningrad, and a slight aperture in this "Window on the West," built as St. Petersburg on a marsh in the early 18th century by Peter the Great.

The signals of the new trend are everywhere.

In April, Pan American World Airways is to begin twice-weekly landings at Leningrad, the first air links between the United States and the Soviet Union's second-largest city.

The Hermitage Museum, in the heart of the city, will open a display of Impressionist paintings from Washington's National Gallery in three weeks, the first art show from the United States in this country in six years.

A visit here by the San Francisco Philharmonic, and exchanges of ballet troupes between the world-famous Kirov and American companies are also under discussion, according to western diplomats here.

The general strain in U.S.-Soviet relations that followed the 1979 Soviet invasion of Afghanistan took an added twist here.

Tensions between local authorities and the small contingent of Leningrad-based U.S. diplomats mounted after incidents in the spring of 1984 in which a marine and a U.S. diplomat were roughed up by locals.

The State Department then issued a travel warning to Americans to avoid Leningrad.

As a result, the 1983 level of 45,000 American tourists fell to 35,000 in 1984, according to western figures.

After the warning was lifted last March, figures rose again and are still on the way up. This year, western authorities say, 55,000 to 60,000 Americans are expected to visit Leningrad -- a higher total than in any year since detente came to an end in 1979.

The Window on the West has also provided an outlet for a small number of Leningrad residents with spouses in the United States.

Six of the Soviet citizens who have been identified by Soviet officials as eligible to join family members in the United States come from Leningrad, according to diplomatic sources here. They include Mikhail Iossel, married to Edith Luthi of Holliston, Mass., and Leonid M. Ablavsky, married to Robin Rubendust of Somerville, Mass.

In addition, Soviet officials told San Francisco Mayor Dianne Feinstein that 10 of the 80 cases of so-called divided families cases she raised during a visit here last month would be settled.

Links between Leningrad and San Francisico -- a potential sister city -- were strengthened during Feinstein's three-day visit here, which was heavily publicized in the local press.

The link would be the first between a U.S. city and Leningrad, which is already involved in cultural and other exchanges with 22 foreign cities, including Manchester, England, and Milan, Italy.

San Francisco often has been compared to Leningrad since 1972, when the United States opened a consulate here and the Soviet Union opened one in San Francisco.

But the political costs of aligning with a city with a reputation among human rights activists here and in the United States for tough treatment of Jews and dissidents may be high, western diplomatic sources here feel.

Since the November summit in Geneva between Soviet leader Mikhail Gorbachev and President Reagan, the record in Leningrad has been mixed.

Although some Jews and dissidents have received permission to emigrate, others report a crackdown on Hebrew teachers and students.