Israeli commandos shot passengers at the rate of one a minute during the bloody raid on an aid flotilla bound for Gaza, a Briton on board the main vessel said.
Ismail Patel, chairman of Leicester-based Palestinian rights group Friends of Al Aqsa, said one of the nine people killed during the assault was shot just two feet in front of him.
He claimed the commandos had a "shoot to kill" policy during the initial phase of the attack on the Mavi Marmara, with live rounds being fired on the vessel from a helicopter hovering above.
Mr Patel, who returned to the UK after attending the funerals of his shipmates in Turkey, told a press conference in Westminster the gunfire on the vessel last around an hour. Despite Israeli claims of armed resistance, Mr Patel said those on board acted in self-defence using whatever was at hand.
Mr Patel said: "We now can calculate that they shot one person every minute. One person was shot every minute. There were nine fatalities, over 48 people with gunshot wounds, six are unaccounted for."
Mr Patel said the initial assault was mounted from the water, with sound bombs, tear gas and stun grenades fired against the Mavi Marmara, but people on the vessel threw items overboard to repel the attack. Then the helicopter appeared overhead "and started using immediately live ammunition" without any warning being issued. After the first victim fell the white flag was raised, Mr Patel said, but Israeli forces continued firing and soldiers rappelled to the deck on ropes.
As the death toll mounted Mr Patel used the ship's public address system to call for a surrender. "I spoke in English very calmly and said 'we are only civilians, we surrender' and requested my colleagues to sit down in their chairs, put their hands and legs on the tables and not to make any movement." But, he said: "Despite our repeated calls the firing continued."
Once the troops had the passengers surrounded the vessel was diverted to an Israeli port and Mr Patel and his colleagues were detained in prison. Alex Harrison, 32, from Islington, North London, was on one of the smaller vessels in the flotilla and witnessed the Mavi Marmara being stormed.
Ms Harrison, from the Free Gaza Movement, said: "I have seen some selective footage that the Israelis have chosen to put out suggesting that we responded with violence. You must remember that these are unarmed civilians on their own boat in the middle of the Mediterranean. People picked up what they could to defend themselves against armed, masked commandos who were shooting."
Foreign Secretary William Hague confirmed that a total of 34 of the activists on the aid flotilla were British, with all but two of them having been sent to Turkey by the Israeli authorities. Israel has previously said its troops had been left with no choice after they came under attack from activists armed with knives and iron bars when they were dropped by helicopter onto the ship.