If you saw the Warren Beatty movie ''Reds,'' you'll remember the spectacular scene when crowds invade the square in front of the Czar's Winter Palace in Petrograd to set off the Russian Revolution of October 1917.
If you go to see ''Gorky Park,'' the thriller about the KGB to be released in December, you'll see skaters in that famous park by the Moscow River. And there'll be a Kremlin tower in the background.
Only the square wasn't in Petrograd (now Leningrad) and the park and tower won't actually be in Moscow. They are here in Helsinki.
Confused? Read on.
''Gorky Park'' is actually Kaisaniemi Park, just north of the Helsinki railroad terminal - although some scenes for the film were shot in Lapland, because Helsinki was uncharacteristically short of snow and ice last winter.
And the ''Kremlin tower'' is really that of the Finnish National Museum. Out of view, in the street behind the museum, a giant red star is hoisted just above the peak of the tower by a hydraulic lift.
For years, Finland has proved a perfect stand-in for motion pictures about the Soviet Union.
The two major movies above follow ''Telephon,'' with Charles Bronson, ''The Billion Dollar Brain,'' with Michael Caine and Karl Malden, ''Coming out of the Ice'' (a made-for-television movie that ran on CBS), and some scenes in ''Dr. Zhivago'' (which supposedly take place in Siberia) in using Scandinavian settings to simulate locations across the border.
But suddenly the Finns are not as pleased as they once were to lend their streets and buildings to movie companies eager for ''Moscow'' and ''Leningrad'' settings. Not long ago the Helsinki real estate committee broke precedent and refused a Western production company permission to use Helsinki for a new film.
The reason goes to the heart of Finland's delicate, careful, complex relationship with its giant neighbor - and to its own constant search for identity in the shadow of the bear.
The movie was to be about the life of Dr. Andrei Sakharov, father of the Soviet H-bomb, who turned against the Soviet system and remains the most famous symbol of dissidence to the world at large. Sent into internal exile in the city of Gorky in January 1980, he remains there in precarious health with his wife, Yelena Bonner.
Through the Finnish-Soviet Friendship Society, which reflects official Moscow's views, the Kremlin made it clear that it disapproved of the planned film. It spoke of a ''risk'' that ''tendentious and anti-Soviet films may distort our foreign policy status (with Finland).''
Finnish officials, caught between wanting to assert Western freedom and the need for unruffled ties with its neighboring superpower, conceded that the idea posed particular problems in the very city where the Helsinki Final Act on European security and human rights was signed in 1975.
In the background lies a surprising amount of official irritation that has been building in Helsinki because of the way Finland has been used as a Soviet stand-in.
''Frankly, we don't like Helsinki always being used as a 'little Moscow' or 'little Leningrad,' '' commented a senior Foreign Ministry diplomat in an interview.
''Why can't Helsinki be used as the capital of Finland for a change? Or as a typical northern city, or Nordic one? Why must we masquerade all the time as something we are not?''
The obvious answer, according to several Finnish film producers who worked on both ''Reds'' and ''Gorky Park,'' is that Hollywood and Europe are much more interested in movies set in Moscow than in Helsinki.
''Helsinki is a good town to portray Moscow and Leningrad location scenes,'' says Kaj Holmberg, managing director of Scandia Films Company and the production manager for ''Reds.''
''It has the buildings in Government Square designed by the same architect who worked in Leningrad. It is compact. The older parts are relatively easy to seal off; you just have to redirect a few trams.
''You have men standing by to take down television aerials - we retune (them) when we put them back, so people don't really mind - and another to paint out pedestrian crossings, then paint them in again. . . . It's not too bad.''
Also, as the diplomat conceded, many Finns enjoy the glamour of a major movie set. They like reading about it all in the newspapers, seeing footage on the television news, and even volunteering for crowd scenes.
And the country makes money on the deal. ''Reds'' and ''Gorky Park'' pumped millions of dollars into Finland during months of filming.
Yet ''the shutters are down now,'' in the words of one Finnish editor.
Trouble has been brewing, in fact, since the CBS TV film, ''Coming Out of the Ice.'' Mr. Holmberg says flatly that the Los Angeles company that made the film under contract for CBS has failed to pay all its bills, and that ill feeling lingers.
''Reds'' went well, according to second assistant director Lauri Torhonen. Its main character, John Reed, played by Beatty, is a hero in the USSR because of his presence at the 1917 October Revolution and his book about it, ''Ten Days That Shook the World.''
Moscow had no objection in principle to Beatty's filming in Moscow and Leningrad. But red tape and delays were so formidable that Helsinki was the logical alternative.
''Gorky Park,'' however, raised Soviet hackles.
At diplomatic receptions during the filming, Soviet diplomats would ask ranking Finnish diplomats, ''Have you read this book?'' When the Finns replied that it seemed a good thriller story, the Soviets reportedly would reply, ''Well , we take a different view.''
The Foreign Ministry, emphasizing that Finland is a free country, did not stop producer Howard Koch, although it told him privately he would be wise to avoid seeking permission to use official buildings.
''Just go ahead and use private buildings where you can,'' Koch was told. ''Don't ask us to say anything in public.''
The filming became a conversation piece throughout the country.
Film crews swarmed. Big red ''M'' signs were put up to simulate Moscow metro stations. A row of downtown stores was transformed into drab Soviet shops. (One, a flower shop in the center, demanded 10,000 Finnish marks - $1,800 - and got it , while the other shops were paid a mere $180 each.)
With every burst of publicity, Soviet irritation grew.
So when the movie about Mr. Sakharov was planned, the the men in the Kremlin had had enough. They exerted indirect pressure.
Officially, the Helsinki real estate committee objected to a decision by its staff to charge Titus Productions Ltd. a mere 1,000 Finnish marks to use locations here. (''Gorky Park'' was charged only 500 marks.)
In fact, the reason was diplomatic.
What of the future?
''We don't want to create the impression that wherever a film about Moscow is to be made, Helsinki is used,'' said the Foreign Ministry diplomat. ''But if we put our foot down every time, we'll get a lot of bad publicity.
''We haven't closed the door to more films, but . . . how about a film about sunny Helsinki, capital of sunny Finland? If you want Moscow, why don't you build a Moscow set in Hollywood?''