"Instead of responding to the findings of the report, Israeli officials are trying to discredit the report and Human Rights Watch by making false allegations," said HRW, which documented what it says are the cases of 11 Palestinian civilians shot dead even though they were waving white flags.
"Instead of seriously addressing the findings of human rights groups in Gaza, the Israeli government is waging a propaganda war against them," said HRW program director Lain Levine.
"If the Israeli government wants to silence critics, it should fully investigate allegations of wrongdoing and take action to end the abuses," he said.
The Israeli military has dismissed the report published on Thursday saying it was based "upon the testimony of a number of Palestinians whose credibility has not been substantiated."
The New York-based watchdog insisted its research did not rely only on witness accounts, but also on medical reports, forensic evidence, interviews with military, medical and other officials and, where possible, the perpetrators.
The office of Israel's Prime Minister Benjamin Netanyahu on Thursday said HRW's recent fundraising activities in Saudi Arabia "in tandem with the kingdom's authoritarian government" raised questions as to the organization's credibility.
HRW acknowledged it visited Saudi Arabia in May 2009 and attended two private receptions, whose 50 guests included three people "with governmental affiliations."
It said the visit was part of an effort to broaden its funding base, but stressed it did not solicit and would never accept funds from the government.
"These events have no bearing on our extensive track record of investigations into Saudi Arabia's appalling human rights record, most recently on August 10, documenting abuses against Saudi detainees.
"Predictably, we have been charged with bias against Saudi Arabia, just as we have been charged with bias against Israel," HRW said.