The Greek coastguard intercepted a boat carrying U.S. activists soon after they set sail from Greece on Friday to join a pro-Palestinian flotilla to Gaza, one of the activists said.

The boat, dubbed "The Audacity of Hope," had just left Perama port, near Piraeus, for the open seas, a Reuters witness said.

A flotilla of boats planning to challenge Israel's naval blockade of the Gaza Strip is expected to sail next week, after repeated delays that activists blame on Israeli sabotage. Israel has denied the accusations.