Tel Aviv, Sept. 28 -- Mosab Hassan Yousef, the son of a senior Hamas leader who later became an Israeli intelligence asset, has called on Israelis to mount a counter-flotilla in response to an international effort to break the blockade of Gaza.

Yousef, who once worked undercover for the Israel Security Agency (Shin Bet) has since become a vocal advocate for Israel abroad, issued his appeal over the weekend in a series of fiery social media posts.

"Listen up, Israeli boat owners--sea wolves, fathers, mothers, sons, daughters," he wrote. "Fifty boats draped in fake flags, packed with terrorist sympathizers, sail to slander us while our hostages rot. This isn't a protest; it's a thunderclap. We don't debate. We sail."

Yousef urged citizen...