A flotilla of nine vessels and several hundred activists have said they want to break Israel's sea blockade and deliver aid to the Palestinian territory.
Greece's Civil Protection Ministry said coast guard authorities had been ordered to take "all appropriate measures" to implement the ban. It also said the "broader maritime area of the eastern Mediterranean will be continuously monitored by electronic means for tracking, where applicable, the movements of the ships allegedly participating" in the flotilla.
Witnesses said one of the boats, dubbed the Audacity of Freedom and carrying several dozen Americans, had left port Friday afternoon. Activists said on their Twitter feed that they had been intercepted by the Greek coast guard. Greek authorities did not immediately comment on the report.
Israel has said it will thwart any effort to breach the sea blockade of Gaza, which was imposed on the Palestinian territory after Hamas militants overran it in 2007.
An Israeli raid on a similar flotilla last year killed nine activists on a Turkish ship and each side blamed the other for the violence.
