Stockholm — Little neutral Sweden (population 8.5 million) captured a Soviet submarine and spent Oct. 29 wondering what on earth to do with it. A drama that at times bore a remarkable resemblance to an old Peter Sellers film, ''The Mouse That Roared,'' began when the submarine went aground the night of Oct. 27 on a rock just eight nautical miles off Sweden's main naval base on the Baltic at Karlskrona.
It hit the rock with such a bang it woke up inhabitants on nearby islands, but nobody bothered to report it because they assumed it was one of the frequent Swedish naval exercises held in the area.
It was not until the next day that fisherman Ingvar Svensson spotted the submarine, of a type designated by NATO as ''Whiskey'' class twin screw high and dry in the early morning Baltic mist.
The badly scared Svensson headed for his mink farm on the island of Sturko and phoned the Swedish Navy. ''I thought at first I was dreaming. It put the fear of God into me when I realized it was a Russian sub,'' he told me.
The Swedish Navy went into action, somewhat crestfallen over the fact that a Soviet submarine had spent the night inside one of its most top-secret, restricted areas.
Torpedo boats surrounded the submarine and a helicopter hovered overhead.
A shame-faced Russian captain, Lt. Comdr. Sacha Guschin emerged from the conning tower and explained over a loudspeaker in halting German that he had got lost after suffering radar failure on a voyage from his home base at Tallinn in Estonia.
Swedish Navy Comdr. Karl Andersson expressed skepticism over his explanation, bearing in mind that the Soviet sub had sailed through a complex series of islands and skerries and through the narrow channel entrance to Karlskrona.
Andersson told Guschin he was inside a restricted military area. He was allowed on board the Soviet sub.
He said afterward that both Guschin and his 50 to 60 crew members were ''extremely depressed.''
Asked what fate awaited him on his return to the Soviet Union, Guschin drew the edge of his hand across his throat, said Andersson.
It was at this point that Guschin told the Swedish commander that he had radioed for help and the Soviet Navy was on its way.
This set off a flurry of diplomatic activity in Stockholm. Soviet Ambassador Mikhail Yakovlev was called to the Foreign Ministry and ''informed'' of the incident.
As darkness gathered, reports reached Karlskrona that a fleet of four Soviet vessels was approaching rapidly from the east.
Yakovlev was hastily summoned back to the Foreign Ministry, where Foreign Minister Ola Ullsten handed him an official protest note describing the incident as a ''flagrant violation'' of Swedish territorial rights.
Defense Minister Torsten Gustafsson the armed forces' chiefs of staff: ''This is the most serious violation of Swedish neutrality since the Second World War.''
The Swedish Navy threw a cordon of ships around the submarine, beamed searchlights on it, and issued instructions that no Soviet ships were to be allowed into Swedish waters and that no one was to leave the sub.
The Soviet asmbassador's request that Soviet vessels be allowed to salvage the sub was curtly refused by Foreign Minister Ullsten. The ambassador was also told that the Swedish Navy would salvage the stricken sub, which was leaking oil and badly dented but otherwise seaworthy.
Guschin and his dispirited crew then spent their second night on the rocks as four Soviet ships - a salvage vessel, a tow boat, and two destroyers - dropped anchor just outside Swedish waters.
The Swedish Navy was put on alert. More ships were sent to the area from Stockholm.
After an uneasy night, during which more Soviet ships were reported to be heading toward Swedish waters, Swedes woke up Oct. 29 faced with having to come to terms with what had happened.
State Prosecutor Magnus Sjoberg said he was considering prosecuting Guschin and his entire crew for espionage.
Prime Minister Thorbjorn Falldin said: ''There is absolutely no question of Soviet vessels being allowed to salvage the submarine.''
Asked what Sweden intended to do, he said: ''We have a right to defend our territory and we shall do this according to international law.''
There was speculation in Stockholm that Guschin was on a spying mission ordered by Moscow because of Soviet distrust of Swedish neutrality.
This has been sparked by a four-day visit to Sweden earlier this month by US Defense Secretary Caspar Weinberger.
Both the Soviet news agency Tass and the Communist Party newspaper Izvestia have since condemned the visit and accused the US of pressuring Sweden to move closer to NATO.
As the Swedish Cabinet went into emergency session to decide what to do with the submarine, it was revealed that a second unidentified submarine had been located inside Swedish waters in the Karlskrona area and was under surveillance by Swedish naval units. A shock charge had been dropped to warn it away, the Navy said.
The Swedish commander in chief of the armed forces, Gen. Lannart Ljung, said he was canceling an official visit to the Soviet Union because of the unexpected tension suddenly arising between Sweden and the Soviet Union.
Following the Swedish Cabinet meeting it was announced that orders had been given for the Swedish Navy to salvage the submarine.
''After an investigation, it will be decided when and how the boat shall be returned,'' said an official communique.
It left a lot of questions unanswered. Such as what will happen to Guschin and his crew. Preparations were already under way in Karlskrona to take the Russians ashore, according to the Swedish News Agency, Tidningarnas Telegrambyra.
Not to mention what the Soviet Union will do when it learns of the Swedish decision. Stockholm, unused to such international drama, held its breath and waited.