Members of a Tokyo-based political group flew to Ishigaki Island in Okinawa Prefecture on Wednesday in preparation for a boat trip to the disputed Senkaku Islands in the East China Sea.
The activists said they planned to set sail on a fishing boat around midnight Wednesday and arrive at Uotsuri Island on Thursday morning.
However, the Japan Coast Guard said it would stop them from leaving the port at Ishigaki
The move comes after the deportation March 26 of seven Chinese activists who were arrested for landing on Uotsuri Island two days earlier.
The move by the Japanese activists may rekindle protests by China and Taiwan, both of which also claim sovereignty over the uninhabited islands that Japan controls.
The coast guard said it would ask the activists to abandon their plan and will stop them in accordance with the Japan Coast Guard Law if they try to set off for the island.