A "Freedom Flotilla" aimed at delivering aid to Gaza was blocked in Turkey on Saturday after being denied use of two of its ships, which organisers blame on Israeli pressure.
The coalition of NGOs and other associations said it was unable to set sail after the West African country of Guinea-Bissau withdrew its flagged vessels.
"Sadly, Guinea-Bissau has allowed itself to become complicit in Israel's deliberate starvation, illegal siege and genocide of Palestinians in Gaza," the Freedom Flotilla Coalition said.
"The Guinea-Bissau International Ships Registry (GBISR), in a blatantly political move, informed the Freedom Flotilla Coalition that it had withdrawn the Guinea Bissau flag from two of the Freedom Flotilla's ships, one of which is our cargo ship, already loaded with over 5,000 tons of life-saving aid," their statement said
The group said the Guinea-Bissau authorities made several "extraordinary" requests for information including destinations, potential additional port calls, cargo manifest, and estimated arrival dates and times.
"Normally, national flagging authorities concern themselves only with safety and related standards on vessels bearing their flag," it said, equating it to being asked about destinations when registering a car.
At an Istanbul press conference, about 280 volunteers - activists, lawyers and doctors - who had hoped to join the ships shouted slogans including "Flag the flotilla", "We will sail" and "Free Palestine".
Additionally, Hamas's armed wing released footage of two hostages - including one Israeli-American national. The released clip comes as Hamas revealed that it is "studying" Israel's latest truce proposal, as mediators attempt to revive ceasefire talks between the two.