An Italian pro-Palestinian activist was arrested with a Briton and an American in the waters off the Gaza Strip on Tuesday as the Israeli navy stopped a protest by local fishermen against the reduction of their fishing limits.
Vittorio Arrigoni and the other two activists, who were not identified, are being questioned by border police.
A military spokesman said they were known for their contacts with Palestinian terrorist groups.
The International Solidarity Movement (ISM), the organisation the activists belong to, said the three were on the three boats to film possible incidents with the Israeli navy.
The fishermen are protesting the reduction of the waters where Palestinians can fish.
The limit was set at 20 nautical miles (37 km) by the 1993 Oslo accords but has since been gradually cut to six nautical miles (11 km).
There have been frequent incidents between fishing boats and the navy outside the 11-km zone.
