A Swedish peace organisation hopes to defuse tensions with Russia by installing an underwater sign that broadcasts the Morse code message, "This way if you are gay."

The Swedes have come up with an unexpected way of repelling unwanted Russian submarines.

It's called the "Singing Sailor", and it has just been lowered into the sea in the archipelago near Stockholm. It is a waterproof box about a metre high, containing a neon sign with a sailor in his underpants, above the message "Welcome to Sweden – Gay since 1944".

The box also contains a sonar system that transmits – in Morse code – "This way if you are gay".

Sweden legalised homosexuality in 1944. Russia, however, is a country of institutionalised homophobia. Recent new laws banned gay "propaganda" and anti-gay vigilantes and hate crimes are barely discouraged.

Though homosexuality has been decriminalised since 1993, the Kremlin and its Orthodox church allies have increasingly painted gay culture as undesirable evidence of Western decadence.

The Singing Sailor is a joke with a serious point – not only about Russian oppression, but on the home front as well.

The Swedish Peace and Arbitration Society, which was behind the stunt, said its purpose was to urge the Swedish government "to think in new ways instead of falling back on territorial defence, conscription and rearmament – the world doesn't need any more weapons."

"If military actions and weapons had functioned as conflict-resolution methods there would be peace in the world a long time ago," SPAS President Anna Ek said.

In September last year, two Russian fighter bombers reportedly entered Swedish airspace, prompting fury from the locals, and the next month the Swedish military claimed a foreign submarine had been patrolling Swedish waters.

In April this year, Finland fired a depth charge in a warning to a suspected submarine in its waters near Helsinki.

Last month, Swedish security police listed Russia as the biggest security threat to the country.

Last Friday, Sweden and its Nordic neighbours announced a new plan to extend military co-operation in response to increased Russian military activity.

Neither Sweden nor Finland are NATO members.

The defence ministers of Sweden, Norway, Denmark, Finland and Iceland signed a statement agreeing to share defence intelligence and plan new joint military exercises.

"The Russian aggression against Ukraine and the illegal annexation of Crimea are violations of international law and other international agreements," the statement said. "Russia's conduct represents the gravest challenge to European security."

"The Russian military acts in a provocative manner along our borders and there have been several border violations around the Baltic countries. Russian propaganda and political manoeuvring contributes to sowing discord between the countries and inside organisations such as NATO and the European Union."