ASHDOD, Israel — Israel on Thursday unveiled a massive makeshift detention center in the country’s main southern port and announced the end of days of intense naval maneuvers, vowing to stop a flotilla of hundreds of pro-Palestinian activists trying to break a 3-year blockade of the Gaza Strip this weekend.

Military authorities said that masked naval commandos would greet the eight ships deep out at sea, escort the vessels to port and give each of the activists a stark choice: leave the country or go to jail.

But the tough response threatened to backfire by breathing new life into the activists’ mission and drawing new attention to the oft-criticized blockade of Gaza.

“If the Israelis want us to pay a price, we will pay a price, but we will come again and again,” said Dror Feiler, 68, an Israeli-born Swedish activist.