LENINGRAD, U.S.S.R. (AP) _ Nancy Reagan raced through the palaces of this czarist-built capital Tuesday, declaring the vast art collection at the Hermitage ''staggering'' but lamenting the ''monstrosities'' of modern-day high-rises that blocked the view of the famous skyline.

Mrs. Reagan said she found the Soviet people ''very warm and very open.''

But as for Raisa Gorbachev, ''Well, she's a .... everybody's different,'' Mrs. Reagan said with a shrug.

Tens of thousands of waving, smiling citizens clustered along the main thoroughfares, Nevsky and Moscow Prospekts, as her two-dozen car motorcade slowed to take in the array of columed buildings, domed churches and wide squares and statues that adorn the imperial, inner city area.

And as she admired the maze of boulevards and buildings, she lamented the ''monstrosities'' of modern-day high-rises erected on the edge of the city.

''Staggering, staggering, there's just too much to digest,'' the first lady exclaimed at the end of her five-hour tour of this city on the Baltic, founded by Peter the Great in 1703 and embellished by czars and czarinas over the decades.

Queried whether she had any more sympathy for communism after viewing the extravagent finery used by the imperial family before the revolution, she replied quickly, ''No.''

The outpouring of affection that surrounded Mrs. Reagan appeared to be spontaneous. Officials had not released her route in advance and people lined the main streets to catch a glimpse of her. She appeared as a speck of yellow in the back of a large black limousine.

Asked by American reporters during the flight here about her hand-holding with Raisa Gorbachev at earlier meetings this week, Mrs. Reagan replied, with a shrug of her shoulders, ''I don't know. It just sort of happened.''

Although Mrs. Gorbachev declined to join Mrs. Reagan's tour, she arranged for Lidiya Gromyko, the wife of Soviet President Andrei Gromyko, to accompany her.

The first lady said she found Mrs. Gromyko ''very nice, very friendly, very warm.''

But asked if she had the same reaction to her tours with Mrs. Gorbachev, Mrs. Reagan smiled, and sat silent 12 seconds pondering her answer.

''Well, she's a .... everybody's different,'' she finally said, evoking laughter from the reporters.

When someone asked if Mrs. Gromyko lectured as much as Mrs. Gorbachev, Mrs. Reagan joined in the uproarius laughter. ''Just between us?'' Mrs. Reagan asked, turning to one reporter. ''How many viewers and readers do you have?''

After another long pause, she tried to drop the subject, pleading, ''I'm desperately trying to think 3/8''

Mrs. Reagan and Mrs. Gorbachev will get back together on Wednesday to view icons in the famous Tretyakhov Gallery in Moscow.

Mrs. Reagan started her tour, as most foreign dignitaries do, by paying homage to the 650,000 Russians who starved to death during the 900 day siege of the city by the Nazi invaders in 1941. More than a million people died in the defense of the city during the war.

In a solemn ritual, Mrs. Reagan placed a bouquet of red carnations at the austere, dark stone Monument to the Heroic Defenders.

From there she traveled to the Hermitage, whose art collection - one of the most fabulous in the world - was begun in 1764 by Catherine the Great. With a lecturer from the museum whispering into her ear, Mrs. Reagan was whisked by masterpieces by Leonardo da Vinci, Caravaggio, and other Italian, Flemish, Spanish and French artists.

''Oh, so much to see 3/8'' she exclaimed at one point, dramatically clapping both hands to her head.

The Mrs. Gromyko, who had some trouble negotiating the many stairs and hectic schedule, was heard to observe of the American pace, ''So quick. They stand, they go...'' Asked if she were tired, she said, ''Of course, I'm tired. I'm 77 years old. How could I not be tired?''

The final leg on Mrs. Reagan's tour was a trip by hydrofoil to Peter the Great's summer palace. She was accompanied by Dmitri Likhachev, a famous Soviet specialist in early Russian literature who has backed the preservation of older buildings.

As the boat passed a series of complexes of austere, high-rise apartment buildings, Likhachev commented that they were ''impersonal'' to which Mrs. Reagan agreed. He mentioned that the buildings now blocked a view of the city's famous skyline.

''Progress doesn't always mean something wonderful,'' Mrs. Reagan observed, gesturing towards the buildings. ''But what are we going to do about those monstrosities?''