"Significant prima facie evidence indicates that serious violations of international humanitarian law as well as gross human rights violations occurred during the military operations of 27 December 2008 to 18 January 2009, which were compounded by the blockade that the population of Gaza endured in the months prior to Operation Cast Lead and which continues," the former South African high court judge said.
At least 1,400 Palestinians were killed during Israel's 22-day offensive on Gaza, the Palestinian Centre for Human Rights says.
At least thirteen Israelis, three of them civilians, were killed.
Rights violations included torture, extra-judicial execution, forced eviction, arbitrary detention, home demolition and other violence and restrictions on freedom of movement and expression, Pillay said.
"While these violations are of deep concern in their own right, the nearly total impunity that persists for such violations (regardless of the responsible duty bearer) is of grave concern, and constitutes a root cause for their persistence," Pillay said.
She said that Israel's blockade of Gaza and restrictions of movement in the Palestinian West Bank should be lifted, and that illegal expansion of Israeli settlements in occupied territory should end.
Pillay said that Israel's military justice system for tackling violations did not meet global standards and that independent investigations into allegations of abuses of international law and human rights during the Gaza offensive should be held.
The 34-page report is the first of two ordered by the UN Human Rights Council in January.
The other, written by Richard Goldstone, a South African journalist who has been conducting hearings in Gaza, is to be delivered in September.
Findings of the UN Human Rights Council are not binding, unlike the rulings of the UN Security Council.