One main boat carrying around 20 people accompanied by a group of smaller ones departed from the fishermen's port in Gaza City, AFP journalists reported and organisers said.
![Vladimir Putin has dominated Russian politics for a quarter of a century [Getty Images]](data:image/svg+xml;charset=utf8,%3Csvg%20xmlns%3D'http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg'%20width%3D'100'%20height%3D'100'%20data-ring-placeholder%3D'1'%3E%3C%2Fsvg%3E "Vladimir Putin has dominated Russian politics for a quarter of a century [Getty Images]")
There were conflicting statements about whether the boats would seek to break Israel's blockade, which currently allows them to travel nine nautical miles (16 kilometres) off the coast.
Some organisers said they would only protest, while other participants said they wanted to move past the limit.
It was not clear how Israel's military would respond and it was yet to comment on the protest.
Earlier Tuesday, Israel's army said some 28 mortar shells were launched toward Israel from the Gaza Strip, with most intercepted by the country's air defence system and no injuries reported.
One mortar shell exploded near a kindergarten building, a military spokesman said. No children were believed to be there at the time.
It appeared to be the largest such barrage from Gaza targeting Israel since a 2014 war.
Israeli Prime Minister Benjamin Netanyahu pledged to respond "powerfully" to the mortar fire.
The Gaza Strip has been under Israeli blockade for more than a decade, with Israel saying it is necessary to prevent the Palestinian enclave's militant Hamas rulers from obtaining means to attack.
The boat protest comes after weeks of deadly demonstrations and clashes along the Gaza-Israel border, beginning on March 30.
The protests have called for Palestinians who fled or were expelled in the 1948 war surrounding Israel's creation to be allowed to return to their former homes now inside Israel.
They peaked on May 14, when at least 61 Palestinians were killed as tens of thousands of Gazans protested and clashes erupted on the same day of the US transfer of its embassy in Israel from Tel Aviv to Jerusalem.
Demonstrations and clashes have continued at a low level since then. At least 121 Palestinians have been killed by Israeli fire in the unrest.
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