Israel's navy seized a boat on Tuesday that had left from a Gaza port with wounded Palestinians aboard in a bid to challenge the blockade of the Hamas-run enclave.
Hundreds of people had gathered to see off the boat carrying nine passengers, including four people said to have been wounded by Israeli soldiers along with students.
The passengers "with specific needs are prevented from travelling, receiving care and finishing their studies", said organiser Raed Abu Dair.
"We are determined to break the blockade."
Mahmud Abu Ataya, 25, who recently suffered a leg wound east of Gaza city, said: "I am leaving to be cared for abroad."
But the boat was seized by Israel's navy a short time after setting off.
"After the boat and the Palestinians on board are searched, the boat will be towed to the Israeli navy base in Ashdod," Israel's military said in a statement.
"The [military] assigned medical personnel to treat Palestinians on board requiring medical assistance."
A similar attempt was made in May and also resulted in Israeli forces seizing the main protest boat several kilometres out to sea.
Israel's blockade restricts Gazan boats to up to six nautical miles off the enclave.
Israel and Islamist movement Hamas, which runs the Gaza Strip, have fought three wars since 2008.
UN officials and rights activists have repeatedly called for Israel to lift the blockade, citing deteriorating humanitarian conditions.
On Tuesday, UN Special Coordinator Nickolay Mladenov said he was concerned by Israel’s decision to suspend imports and exports with the exception of basic humanitarian supplies through the Kerem Shalom crossing.
"Humanitarian assistance is no substitute for commerce and trade. I urge the authorities to reverse this decision," he said in a statement.
Israel says it is necessary to stop Hamas and other militant groups in Gaza from obtaining weapons or materials that could be used to make them.
On Monday, Israel also closed its only goods crossing with the Gaza Strip in response to weeks of fires at farms caused by kites and balloons carrying firebombs from the Palestinian enclave.
A spokesman for Israel's fire service says 750 fires have burnt 2,600 hectares.
Gazans began launching the kites as part of protests along the border calling for Palestinian refugees to return to their former homes now inside Israel.
Since protests abroke out along the Gaza border on March 30, at least 139 Palestinians have been killed by Israeli fire and more than 4,000 have been wounded by gunfire.
The majority were involved in protests and clashes, but others were seeking to breach or damage the border fence.
No Israelis have been killed.