Activists sailing on dozens of boats attempting to break Israel’s maritime blockade of the Gaza Strip to deliver humanitarian aid say Israeli forces intercepted them overnight Wednesday into Thursday, detaining the crews while the flotilla was sailing hundreds of kilometres from Gaza near the southern Greek island of Crete.

The Global Sumud Flotilla set sail earlier this month from Barcelona. Organisers have said more than 70 boats and 1,000 people from around the world would be participating,...