HELSINKI, Finland (AP) _ Police stopped a 70-year-old American man who intended to march into the Soviet Union and demand its leaders free a West German pilot who was imprisoned for flying a light plane into Red Square.

Police said John Runnings of Seattle was detained as soon as he landed at Helsinki airport late Friday night. To prevent him from carrying out the plan, they said, he would be put on the next flight back to the United States.

Inspector Hannu Siljamaki, at the Interior Ministry's foreigners' office, said police are permitted to stop at the border any foreigner who ''on the basis of his or her activities indicates criminal intentions.''

Runnings had publicly announced he would walk across the Soviet border without a visa and demand freedom for Mathias Rust, a teen-ager who last May flew a small plane from West Germany via Helsinki to Moscow's Red Square.

The flight across hundreds of miles of tightly guarded Soviet air space drew international headlines and led to a shakeup in the Soviet military. After a trial, Rust was sentenced to four years in prison.

Runnings had sought publicity, which indicates ''his intention is to harm Finland's relations to a third country,'' Siljamaki said.

Helsinki Airport police spokesman Pekka Forsten said the American ''will be sent back to where he came from with the first suitable flight.'' He declined to specify which flight Runnings would be put on.

Last week a crew of Finnair, the Finnish state-owned airline, stopped Runnings from boarding a flight to Finland knowing he would be turned back at the airport. ''We wanted to save him from undue trouble and loss of money,'' Finnair spokesman Usko Maatta said.