Israel's "brutally disproportionate and grossly indiscriminate" bombardment of Gaza must be regarded as a war crime, Labour former deputy prime minister John Prescott said.
Lord Prescott said any other country would be made an international "pariah" if it acted in the same way, as the reported death toll from the offensive against Hamas reached four figures.
And he directly compared the situation in Gaza with a concentration camp, suggesting the Nazi Holocaust should "give Israelis a unique sense of perspective and empathy with the victims of a ghetto".
Thousands of pro-Palestinian demonstrators earlier marched through London at the same time as hostilities were paused for 12 hours under an uneasy humanitarian truce.
Foreign Secretary Philip Hammond, speaking after crisis talks with US secretary of state John Kerry and other foreign ministers in Paris, urged both sides to extend the cessation of violence in a bid to stem the loss of life.
But after Israel announced it would observe another four hours and consider a United Nation's plea for a 24-hour halt, Hamas resumed firing rockets at Israel.
Writing in the Sunday Mirror, Mr Prescott said: "I magine a country claiming the lives of nearly three times as many as were lost in the MH17 plane tragedy in less than three weeks.
"A nation which blasted a hospital, shelled and killed children from a gunboat as they played football on the beach and was responsible for 1,000 deaths, at least 165 of them children, in just two weeks.
"Surely it would be branded a pariah state, condemned by the United Nations, the US and the UK. The calls for regime change would be deafening.
"But these howls of protest are muted. The condemnation softened. For this is Israel.
"Israel's hawkish Prime Minister Benjamin Netanyahu trots out the same excuses. Hamas 'militants' in Gaza fired their rockets first. Israel has a right to defend itself. It needs to protect its citizens.
"And he's right on all three counts - but as always with Israel this is not the full story. The military action supposedly targeting Hamas is so brutally disproportionate and so grossly indiscriminate that it makes it impossible not to view Israel's actions as war crimes."
He said Hamas was wrong to fire rockets and should recognise Israel's right to exist, but suggested a violent response was inevitable.
"Israel brands them terrorists but it is acting as judge, jury and executioner in the concentration camp that is Gaza," he said.
"What happened to the Jewish people at the hands of the Nazis is appalling. But you would think those atrocities would give Israelis a unique sense of perspective and empathy with the victims of a ghetto."
He said Labour leader Ed Miliband had shown "true leadership" by condemning the Israeli military incursion, accusing Prime Minister David Cameron of "sitting on the fence".
Speaking after meeting with Mr Kerry along with foreign ministers from other EU nations, Turkey and Qatar, Mr Hammond said they had agreed that a longer truce was needed to prevent further loss of life.
More than 1,000 Palestinians, mainly civilians, are reported killed over the past 18 days while Israel has lost 37 soldiers and two civilians.
"E verybody understands that there is an absolute humanitarian necessity to see this ceasefire extended," he said.
"The necessity right now is to stop the loss of life and you stop the loss of life by getting this ceasefire to roll over for 12 hours, or 24 hours or 48 hours, and then again and again."
The halt in fighting is taking place to allow civilians in Gaza to receive aid and evacuate to safer areas, but there are fears that an upsurge of violence could follow.
Hamas accused Israel of using the pause to prepare for a heightened offensive.
Mr Hammond said he and his counterparts were prepared to commit resources to support the regeneration of Gaza once a resolution had been agreed and the security of Israel had also been secured.
Earlier International Development Secretary Justine Greening announced that the UK is to provide an additional Â£2 million in emergency assistance to the United Nations Relief and Works Agency's (UNRWA) Gaza Flash Appeal.
It brings the amount released this week to Â£7 million, with the money funding basic shelter, blankets, hygiene kits, nappies and other vital supplies for the tens of thousands of people forced to flee their homes.
Mr Hammond yesterday issued a reminder to Israel of its obligations under international law to take "all feasible precautions" to avoid civilian casualties after a UN school came under fire, killing at least 15 Palestinians.