The bloodshed along Gaza's border with Israel is an "abhorrent violation" of human rights, Amnesty International said on Monday after dozens of Palestinians were killed by Israeli fire.
"We are witnessing an abhorrent violation of international law and human rights in Gaza... This must end immediately," the London-based human rights group said on Twitter.
Amnesty gave a death toll of 38 dead as clashes raged along the border with Israel over the transfer of the US embassy from Tel Aviv to Jerusalem.
The death toll reached 52 by Israeli fire, after an early toll of 37 dead, and said over 2000 Palestinians were wounded.
Human Rights Watch's executive director for the Middle East and North Africa, Sarah Leah Whitson, denounced the "bloodbath" in Gaza.
"The policy of Israeli authorities to fire irrespective of whether there is an immediate threat to life on Palestinian demonstrators in Gaza, caged in for a decade and under occupation for a half century has resulted in a bloodbath that anyone could have foreseen," she said.
Amnesty said that more than 500 people had been injured "with live ammunition" and that many were reporting injuries to the head and chest.
"This is a violation of international standards, in some instances committing what appear to be wilful killings constituting war crimes," Amnesty's Middle East and North Africa director Philip Luther said in a separate statement.
"As violence continues to spiral out of control, the Israeli authorities must immediately rein in the military to prevent the further loss of life and serious injuries."
Amnesty made the statement "responding to reports that dozens of Palestinians have been killed" in the protests over the US embassy move.
"The rising toll of deaths and injuries today only serves to highlight the urgent need for an arms embargo," Luther added.
"While some protestors may have engaged in some form of violence, this still does not justify the use of live ammunition."