The ship set sail from the Lebanese port of Tripoli
Israel has expelled activists and journalists on a Lebanese ship carrying aid for Gaza, who were held on Thursday after Israel's navy seized the ship.
Israeli forces beat and kicked some of those on board, said an al-Jazeera reporter on the ship, although Israel said no "acts of violence" occurred.
Israel also denied claims its gunboats had fired on the ship, the latest to try to break the blockade on Gaza.
Israel allowed blood products from the vessel to enter Gaza by road on Friday.
The Israeli military said no weapons had been found on the ship.
Earlier it had said the ship could be a security threat or be used for smuggling banned equipment.
Fifteen people - Syrian and Lebanese nationals - had been transferred to Lebanon and Syria, police spokesman Mickey Rosenfeld told the Associated Press news agency.
Another three, two Indians and one Briton, were in police custody pending deportation, he said.
The former Greek-Catholic archbishop of Jerusalem, 84-year-old Monsignor Hilarion Capucci, was among those sent to Syria, reports said.
The former archbishop had served time in an Israeli jail in the 1970s for smuggling arms for the Palestine Liberation Organisation (PLO).
"The Israeli army confiscated all our videotapes; we were separated from each other, we were blindfolded and handcuffed," al-Jazeera reporter Salam Khader said on the Arab satellite television channel.
"They beat some of us... the soldiers kicked Dr Hani Suleiman [one of the co-ordinators of the ship's mission], in the chest and back."
"During the takeover of the ship, no acts of violence occurred and there was no need to use excessive force," the Israeli military said in a statement.
Activists said the Togo-flagged Tali ship was carrying medical supplies, food, clothing and toys.
Under an Israeli-imposed blockade, only basic humanitarian items - mainly food and medicines - are allowed into the territory.
The flow of goods into Gaza has increased since the recent Israeli military campaign there, but aid agencies say more goods and a wider range of supplies are essential for the strip's recovery.
An organiser of the shipment, Maen Bashur, said the ship had been confronted by an Israeli military boat 32km (19 miles) off the Gazan coast late on Wednesday.
Israeli Defence Minister Ehud Barak said the Israeli navy had requested the ship head towards Egypt, and only boarded it after it headed back towards Gaza.
Israel pulled troops and settlers out of Gaza in 2005, but still controls the strips border crossings, airspace and coastal waters.
It initially allowed a number of similar ships to dock, but more recently has begun turning them back.
This was the first time Israeli troops had boarded a vessel seeking to reach Gaza.
Israel ended a three week military campaign in Gaza on 18 January. About 1,300 Gazans were killed, about a third of them civilians. During the fighting, 13 Israelis were killed, 10 of them soldiers.