An online campaign calling on the public to write letters demanding that Israel not harm the Madleen or the activists on board has racked up more than 498,000 submissions, Al Jazeera reports.

Organised by the Freedom Flotilla Coalition, participants in the campaign are asked to “put Israel on notice” by sending a prepared letter to Israeli officials as well as the UN and foreign governments.

The letter states: “Any interception, attack, sabotage, or interference with the Madleen or its civilian passengers and crew will constitute a deliberate, knowing, and unlawful attack on civilians and a violation of international law.”

The Freedom Flotilla Coalition states on its website: “We are putting them on notice”.

“But this letter alone will not stop an attack on those trying to deliver aid to the Palestinian people of Gaza. Your voice and action are what make this protection real,” it states.