Eyewitness accounts from ships raided by Israeli commandos have cast doubt on Israel's version of events that led to the deaths of at least 10 people.
German pro-Palestinian activist Norman Paech said he had only seen wooden sticks being brandished as troops abseiled on to the deck of the ship.
Israel says its soldiers were attacked with "knives, clubs and other weapons" and opened fire in self defence.
The raid led to widespread condemnation and the UN has called for an inquiry.
The six ships, carrying aid and campaigners, had sailed from Cyprus in a bid to break Israel's blockade of the Gaza Strip.
"This was not an act of self-defence," said Mr Paech, a politician, as he arrived back in Berlin wrapped in a blue blanket.
"Personally I saw two and a half wooden batons that were used... There was really nothing else. We never saw any knives.
"This was an attack in international waters on a peaceful mission... This was a clear act of piracy," he added.
Mr Paech had been a passenger on the Turkish passenger ship Mavi Marmara where most, if not all, of the deaths occurred.
Fellow German activist Inge Hoeger said they had been on the ships "for peaceful purposes".
"We wanted to transport aid to Gaza," she said. "No-one had a weapon."
She added: "We were aware that this would not be a simple cruise across the sea to deliver the goods to Gaza. But we did not count on this kind of brutality."
Activist Bayram Kalyon, arriving back in Istanbul, had also been a passenger on the Mavi Marmara.
"The captain... told us 'They are firing randomly, they are breaking the windows and entering inside. So you should get out of here as soon as possible'. That was our last conversation with him."
Meanwhile, in Nazareth, Israeli Arab MP Haneen Zuabi - who was on the flotilla - told a press conference that Israeli forces began firing while still in the helicopters hovering over the ships.
"We are calling for an international committee to investigate this tragedy," she said.
Diplomatic sources in Ankara have said at least four of those killed were Turkish. Turkish Prime Minister Recep Tayyip Erdogan said the assault was a "bloody massacre" and must be punished. He said Israel should not test Turkey's patience.
UN criticism
Further criticism of Israel came from UN Secretary General Ban Ki-moon on Tuesday.
In an interview with the AFP news agency he said Israel's blockade of Gaza was responsible for the deadly raid.
"Had Israelis heeded to my call and to the call of the international community by lifting the blockade of Gaza, this tragic incident would not have happened," he said.
Nato Secretary General Anders Fogh Rasmussen called on Israel to release people and boats it had seized.
He spoke after an emergency meeting of Nato ambassadors in Brussels called by Turkey.
Renewed violence broke out in Gaza on Tuesday, with five Palestinians reportedly killed by Israeli fire.
Two Palestinian gunmen were shot dead after crossing the border in the south of the territory, a military spokesman said.
Three more people died in an Israeli strike in the north of Gaza, according to Gaza's emergency services. Israel said it carried out an air strike after two rockets were fired from Gaza.
Following the Israeli sea-born raid, Egyptian President Hosni Mubarak ordered the border crossing between Egypt and the Gaza Strip to be opened.
The Rafah crossing has been closed since 2007, although special medical cases are occasionally allowed through.
The Egyptian state news agency said the latest opening was to allow humanitarian aid through. It is not clear how long it will be kept open.
The UN Security Council issued a statement after deliberating through the night.
It said an investigation should be "prompt, impartial, credible and transparent". It also condemned the "acts" which led to the deaths.
