Israeli PM Benjamin Netanyahu has told an inquiry that Israel "acted under international law" when it intercepted a Gaza-bound aid flotilla in May.
He appeared before a commission looking into the deaths of nine Turkish activists during the raid, which sparked an international outcry.
Mr Netanyahu said the Gaza blockade was legal and that Israeli troops only used force when their lives were in danger.
Critics have attacked the investigation's remit as too narrow.
Subsequent international investigations are expected to be more critical of Israeli policy.
The UN and Turkey are due to hold their own inquiries into the raid.
Later on Monday, Mr Netanyahu threatened to pull out of the UN inquiry if Israeli soldiers were asked to testify.
He was reponding to a comment by UN chief Ban Ki-moon who said no agreement had been made to exempt Israeli soldiers from questioning before the panel.
However, the UN has said all along that it is up to the panel to set its own terms of reference, including which witnesses to call.
Addressing the Israeli commission, Mr Netanyahu said: "I am convinced that at the end of your investigation, it will become clear that the state of Israel and the IDF [Israel Defense Forces] acted in accordance with international law."
He said the military action was necessary to prevent weapons getting into the Gaza Strip, and Israeli commandos had taken action only when they believed their own lives were in danger.
Israel says its commandos used live fire only after being attacked with clubs, knives and gunfire by activists.
But activists on board the Turkish-owned Mavi Marmara, where all the killings took place, say the commandos started shooting as soon as they boarded the vessel.
Turkey has described the attack - which took place in international waters, about 80 miles from the Israeli coast - as a violation of international law, "tantamount to banditry and piracy" and described the killings as "state-sponsored terrorism".
Details of the clash remain unclear as the video released by the Israeli military stops just before the shooting begins.
Results of Turkish post-mortem examinations have suggested that a total of 30 bullets were found in the bodies of the nine dead activists, including one who had been shot four times in the head.
Speaking to the BBC, Israeli Arab MP Haneen Zuabi, who was on board the Mavi Marmara during the Israeli commando raid, criticised the enquiry and Mr Netanyahu's testimony. "This was not really a committee to investigate the truth, this was committee to avoid reaching the truth," she said.
In his testimony, Mr Netanyahu said Turkey had rebuffed repeated Israeli appeals to prevent the flotilla from leaving a Turkish port in order to avoid a conflict.
The Israeli inquiry was set up only after international pressure, and some observers say it will not really get to the causes of why the operation went so badly wrong.
Defence Minister Ehud Barak and army chief Gabi Ashkenazi will also be questioned.
Observers say that subsequent international inquiries may be more analytical and critical of Israeli government policy in Gaza.
After criticism from its allies over the flotilla incident, Israel considerably eased its blockade of Gaza - allowing in more food and humanitarian goods.
Israel and Egypt have imposed the blockade on the coastal territory since the Islamist militant group, Hamas, seized control of it in 2007.
The Israelis say it is intended to stop militants in Gaza from obtaining rockets to fire at Israel.
The restrictions have been widely described as collective punishment of the population of Gaza, resulting in a humanitarian crisis.