In Finland he's known as a populist president, and he certainly didn't disappoint the 2,000 Finns and Americans who sandwiched into the Finnish Embassy to meet him last night.

After he edged his way through the jungle of people, shaking hands and exchanging chitchat, Mauno Koivisto, president of Finland, starting popping shrimp. No less than 60 reporters and cameramen followed.

Pop. Pop. Click. Click.

"What's he doing?" shouted an American woman. "I think he's chowing down," explained an American man.

Undaunted by the flashes and eager reporters, Finland's first socialist president, who is considered an anti-establishment politician, kept eating. He fielded reporters' questions in typical George Shultzesque manner: "Not at a party."

This is Koivisto's first official trip here from Finland, the neutral country that shares 800 miles of border with the Soviet Union. His job is one of the world's most taxing--walking the tightrope between Western and Eastern interests.

The president is scheduled to meet with President Reagan this morning. Finnish foreign minister Paavo Vayrynen said Koivisto will probably want to talk to the president about Reagan's speech to the United Nations yesterday in which he called for the United States and the Soviet Union to limit their medium-range nuclear weaponry in Europe. Details of the proposal will be discussed at the arms control talks in Geneva.

"I'm sure also that your president will be interested in our views of the Soviet Union," Vayrynen said. "We have special knowledge . . . Our president has already met with Andropov three times."