Nancy Reagan, welcomed to the "Venice of the North" by tens of thousands of waving and smiling people, marveled Tuesday at the city's art and architectural gems after paying homage to the 650,000 Russians who starved to death in Leningrad.

Mrs. Reagan placed a bouquet of flowers at the Monument to the Heroic Defenders, located near the front line of World War II where the fiercest battles were fought.From there she traveled to the Hermitage, which houses one of the most fabulous art collections in the world. Paintings by Leonardo da Vinci, Picasso, Matisse and other great artists can be found in the collection, begun in 1764 by Catherine the Great.

"Staggering, staggering, there's just too much to digest," the first lady exclaimed at the end of her five-hour tour of the city.

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."

As she began her tour of the museum, Mrs. Reagan was asked about the reception the people of Leningrad had given her.

"Wasn't that wonderful," she said. "I was waving, hoping they would know which car I was in."

The outpouring of affection appeared to be spontaneous. Officials had not released the first lady's motorcade route, which traveled the length of several of the city's renowned boulevards. At some points, people lined up several deep on the broad sidewalks to catch a glimpse of the petite first lady.

En route to Leningrad, Mrs. Reagan told reporters aboard her plane that the Soviet people "are very

warm and very open" but that she was less impressed with her high-level contacts. "Their philosophies and positions are completely different from ours," she said.

Asked about her hand-holding with Raisa Gorbachev, Mrs. Reagan replied, with a shrug of her shoulders, "I don't know. It just sort of happened," indicating that it was an action that she hadn't initiated.

Lidiya Gromkyo, the wife of Soviet President Andrei Gromyko, accompanied Mrs. Reagan on the trip.

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 for a full 12 seconds pondering her answer.

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

The trip to Leningrad provided reporters with their first chance to question Mrs. Reagan about the report by former Chief of Staff Donald Regan that astrology influenced the president's schedule. But she refused to be drawn into any discussion of the matter, declaring, "Everything has been said about that that needs to be said."

But she also promised to tell her own side of the story.