Workers work on the cruise liner Mavi Marmara which is under maintenance in a shipyard in Istanbul, Monday, (Reuters).
"While we wholeheartedly welcome the decision of the Egyptian government to regularly operate the Rafah crossing... Israel's unlawful blockade remains in effect," Vangelis Pisias, the Greek coordinator of the initiative, told reporters.
"Israel still prevents Palestinians from using their sea, and controls and severely restricts all goods entering and exiting Gaza. As such, we must continue to challenge this blockade," he said.
Fifteen ships will leave for Gaza from various Mediterranean ports around June 20, he added.
Some 1,500 activists from about 100 countries will board the vessels, which will also carry humanitarian aid, medical equipment, school supplies and construction materials, including up to 700 tonnes of cement, said Huseyin Oruc, a senior member of the Turkish Islamist charity IHH.
Activists announced their intentions to send a new flotilla to Gaza during a press conference on board the Turkish ferry Mavi Marmara to mark the first anniversary of the bloody ending to the ship's first mission to break Gaza's blockade.
Nine Turkish activists were killed on May 31 last year when Israeli forces raided the Mavi Marmara, which was the lead ship of an international convoy heading to the Palestinian enclave spearheaded by the IHH.
The raid triggered worldwide condemnation and plunged Turkish-Israeli ties into deep crisis.
On Saturday Egypt reopened its Rafah border crossing with Gaza, allowing people to cross freely for the first time in four years and giving Gazans a gateway to the world.
Rafah is the only crossing which does not pass through Israel.
Turkish and foreign activists were to hold a march in downtown Istanbul on Monday evening to commemorate the nine dead in last year's raid.
The boats in the new mission, coming from Canada, France, Greece, Ireland, Italy, Spain, Turkey and the United States, would meet in international waters south of Cyprus before heading to Gaza, Oruc said.