Sweden has stepped up surveillance of key military installations along its coast in the wake of submarine sightings and recently disclosed reports that foreign divers may have penetrated closed military zones in the Stockholm archipelago.
At least one incident of a foreign diver near Stockholm was considered so serious it is being specially investigated by military authorities, says a Defense Ministry spokesman.
Military patrols were prepared to capture or, if necessary, shoot intruding frogmen, the spokesman added.
A military-affairs expert in a major Western capital guessed that if the divers were from a foreign power, Soviet naval special forces ''would be my front-runner.'' He said reconnaissance or training could be the reason for a mission into Sweden.
Apparently in connection with Sweden's role as host to a session of the European security conference, Swedish military authorities have played down the reports of the divers as well as submarine incidents late last fall.
If, as many suspect, the weight of the evidence points to the Soviet Union, it could be embarrassing to Sweden's superpower neighbor. But without clear evidence as to who is behind the submarine incidents and sightings of divers, neutral Sweden could cause itself embarrassment by jumping on the Soviets.
The Swedish Defense Ministry would not give details of the incident, but the Stockholm daily Expressen claimed last weekend that a civilian watchman, considered a reliable witness, had seen strange divers apparently measuring the distance between mines in the Stockholm archipelago last fall.
The divers wore special breathing gear that does not expel bubbles and dark suits rather than the bright colors favored by sport divers for safety reasons. They disappeared as soon as they noticed they were being watched, the Expressen report said.
Undersea mines are an important part of Sweden's defense against invasion and against other intrusions into its waters. The Swedish military purposely detonated mines last year during an unsuccessful hunt for a foreign submarine believed to be in the harbor at Sundsvall north of Stockholm.
The Swedish press has theorized the mysterious divers may have come from minisubmarines, since no vessels were seen near the mine station where the divers appeared. They apparently surfaced to avoid pressure and turbulence from a large ferryboat heading for Finland.
A military spokesman said most of the reports of divers have proved to involve ''normal'' sport divers or to be too fragmentary or doubtful to be taken seriously.
Newspaper commentators have also suggested that rival groups of underwater treasure hunters could be reporting each other to the military and police. But the statement that Swedish patrols have been increased and that they will shoot as a last resort is clear warning, without naming names, to the Soviets and to other foreign powers as well as to domestic mischief-makers.