Amnesty International said Palestinian rocket fire during last year’s conflict with Israel had killed more civilians inside Gaza than inside Israel.
Such attacks on civilians were against international law and constituted "a war crime," it said.
Amnesty urged Palestinian armed groups to end all direct attacks on Israeli civilians and ensure everything was done to also protect Palestinian civilians inside Gaza from the effects of those attacks.
The 50-day war claimed the lives of some 2,200 people in Gaza, most of them civilians, and 73 on the Israeli side, most of them soldiers.
The report said rocket attacks had killed six civilians inside Israel, including a child, but that other rockets aimed at the Jewish state had fallen short inside Gaza, killing at least 13 civilians, 11 of them children.
It referred to one particular incident on 28 July in which 13 people were killed in a blast inside the beachside Shati refugee camp.
At the time, Palestinian witnesses blamed the attack on an Israeli F-16 warplane, but the army denied firing on the camp, accusing militants of misfiring their own rockets.
"In the deadliest incident believed to have been caused by a Palestinian armed group during the conflict, 13 Palestinian civilians - 11 of them children - were killed when a projectile exploded next to a supermarket in the crowded al-Shati refugee camp," the report said.
Contrary to Palestinian claims that Israel was responsible, "an independent munitions expert who examined the available evidence on behalf of Amnesty International concluded that the projectile used in the attack was a Palestinian rocket," it said.
Figures released by the Israeli army just days after the war ended on 26 August showed that Gazan militants fired 4,591 projectiles at Israel.
Of that number, 3,659 struck Israeli territory and 735 were intercepted by the Iron Dome air defence system, leaving another 197 which fell short and landed inside Gaza.
Militant groups used unguided projectiles which were "inherently indiscriminate," it said: "Using such weapons is prohibited under international law and their use constitutes a war crime."
"In launching these attacks, they displayed a flagrant disregard for international humanitarian law and for the consequences of their violations on civilians in both Israel and the Gaza Strip," said Philip Luther, director of the Middle East and North Africa programme at Amnesty International.
He said Palestinian armed groups "must end all direct attacks on civilians" and take all feasible precautions "to protect civilians in the Gaza Strip from the effects of such attacks" including storing weapons or letting militants operate "within or near densely populated areas."
