The Global Sumud Flotilla set sail last month seeking to ferry aid to war-battered Gaza but Israel blocked the boats, detaining more than 400 people whom it began deporting on Friday.

Of that number, 137 activists from 13 countries flew into Istanbul on Saturday, among them 36 Turkish nationals.  

"We were intercepted by a huge number of military vessels," Paolo Romano, a regional councillor from Lombardy in Italy, told AFP at Istanbul airport. 

"Some boats were also hit by water cannon. All of the boats were taken by very heavily armed people and brought to shore," the 29-year-old said. 

"They put us on our knees, facing down. And if we moved, they hit us. They were laughing at us, insulting us and hitting us," he said. 

"They were using both psychological and physical violence."

Among those on board the flotilla, which counted some 45 vessels, were politicians and activists including Swedish climate campaigner Greta Thunberg.

Romano said they tried to force them to admit they had entered Israel illegally. 

"But we never entered Israel illegally. We were in international waters and it was our right to be there."

On landing, they were taken to a prison and held there without being allowed out and were not given bottled water, he said.

"They were opening the door during the night and shouting at us with guns to scare us," he said. 

"We were treated like animals."

Iylia Balqis, a 28-year-old activist from Malaysia, said Israel's interception of the boats was "the worst experience". 

"We were handcuffed (with hands behind our backs), we couldn't walk, some of us were made to lie face down on the ground, and then we were denied water, and some of us were denied medicine," she said.

The activists were flown to Istanbul on a specially-chartered Turkish Airlines plane. 

In a post on X, the Israeli foreign ministry confirmed "137 more provocateurs of the Hamas–Sumud flotilla were deported today to Turkey". 

Relatives of the Turkish activists could be seen awaiting their arrival at the VIP lounge inside Istanbul airport, waving Turkish and Palestinian flags and chanting "Israel murderer". 

The Turkish activists were to undergo medical checks on arrival and would appear in court on Sunday to give testimony, their lawyers said. 

Turkey has denounced Israel's interception of the flotilla as "an act of terrorism", saying Thursday it had opened an investigation.

Foreign Minister Hakan Fidan praised the activists as "brave individuals who gave voice to humanity's conscience" in a post on X and said Ankara would ensure all of its nationals were brought back, without giving an overall number. 

Italian journalist Lorenzo D'Agostino, who was on board the flotilla to cover its mission, said they "were kidnapped in international waters when we were 55 miles (88 kilometres) from Gaza".

"It was two hellish days that we spent in prison. We are out now thanks to the pressure of the international public that supports Palestine," he said.

"I really hope this situation ends soon because it has been barbaric the way we have been treated."  

Libyan activist Malik Qutait said he was not afraid and vowed to keep trying to reach Gaza. 

"I will collect my group, arrange medicine, aid and a ship and I will try again," he said.