Thirty-seven Palestinians were killed by Israeli forces on Monday as tens of thousands protested and clashes erupted along the Gaza border, Gaza's health ministry said, in the deadliest day in the conflict since a 2014 war.
The 37 were killed as huge crowds approached the border protesting against the inauguration of the controversial US embassy in Jerusalem.
The Palestinian Authority government accused Israel of committing a 'terrible massacre' and Palestinian government spokesman Yusuf Al Mahmoud demanded in a statement "an immediate international intervention to stop the terrible massacre in Gaza committed by forces of the Israeli occupation against our heroic people."
It was the highest Palestinian single-day death toll since a series of protests dubbed the "Great March of Return" began at the border with Israel on March 30 and since a 2014 Gaza war.
The health officials said 900 Palestinians were wounded, about 450 of them by live bullets.
Tens of thousands gathered at the frontier on Monday, some of them approaching Israel's border fence - a line Israeli leaders vowed Palestinians would not be allowed to breach. Black smoke from tyres set alight by demonstrators rose in the air.
"Today is the big day when we will cross the fence and tell Israel and the world we will not accept being occupied forever," said Gaza science teacher Ali, who declined to give his last name.
"Many may get martyred today, so many, but the world will hear our message. Occupation must end," he said.
Later in the day, Israeli leaders and a US delegation including Treasury Secretary Steven Mnuchin and President Donald Trump's daughter and son-in-law, Ivanka Trump and Jared Kushner, were due to attend the opening of the embassy relocated from Tel Aviv to Jerusalem.
"A great day for Israel," the US president, who stoked Arab anger by recognising Jerusalem as Israel's capital in December, said in a tweet.
No Israeli casualties have been reported.
The killings have drawn international criticism.
Palestinian Prime Minister Rami Hamdallah accused the US of "blatant violations of international law".
The Palestinians, who want their own future state with its capital in East Jerusalem, have been outraged by Trump's shift from previous administrations' preference for keeping the U.S. Embassy in Tel Aviv pending progress in peace efforts.
Those talks have been frozen since 2014. Other international powers worry that the U.S. move could also inflame Palestinian unrest in the occupied West Bank, which Israel captured along with East Jerusalem in the 1967 Middle East war.
In London, the British government said it had no plans to move its Israel embassy from Tel Aviv to Jerusalem, and said it disagreed with the U.S. decision to do so.
The Russian government said it feared the embassy move would increase tensions across the Middle East.
Turkey on Monday said the United States shared responsibility with Israel for the "massacre" along the Gaza border.
"The US administration is as responsible as the Israeli government for this massacre," Deputy Prime Minister Bekir Bozdag wrote on Twitter, saying the incidents were caused by "unjust and unlawful decisions" as the United States moves its embassy in Israel from Tel Aviv to Jerusalem.
Arabs to discuss 'illegal' US embassy move to Jerusalem
The Arab League will hold emergency talks on Wednesday to discuss Washington's "illegal" decision to move the US embassy in Israel to Jerusalem, a senior official said.
The meeting will focus on "ways of countering the illegal decision by the United States to move the embassy to Jerusalem", the organisation's deputy secretary general for Palestinian affairs, Saeed Abu Ali, said.
He told reporters the permanent representatives of members of the Cairo-based Arab League would meet "at the request of the state of Palestine".
Jordan denounced the embassy move as a "clear violation" of the UN charter.
In a statement, it also "condemned" a unilateral decision by US President Donald Trump to recognise Jerusalem as the capital of Israel, despite Palestinian claims to part of the disputed city.
UN rights experts slam Israel's 'disproportionate use of force'
UN rights experts urged Israel Monday to halt all excessive force against Palestinians and called for an "impartial, independent investigation" into troop violence that has left dozens dead.
The UN Committee on the Elimination of Racial Discrimination said it was "alarmed by the disproportionate use of force displayed by the Israeli security forces against Palestinians."
The independent experts released their statement as almost 40 Palestinians, including a 14-year-old, were killed by Israeli fire on along the Gaza border.
Thousands of others have been injured, with the military facing widespread criticism over its use of live fire.
The 18-member UN committee said it was "gravely concerned that many of the persons who died or were injured were reportedly posing no imminent threat at the time they were shot."
It also voiced alarm at "many reports according to which Israeli authorities have denied and continue to deny access to urgent medical treatment to injured Palestinians."
The experts said the deaths and injuries were occurring "in a context marked by the 50-year occupation of the Palestinian territory" as well as a more than decade-long crippling blockade imposed on Gaza.
They said they were "deeply worried about persisting discriminatory practises against Palestinians by Israel," and lamented "the absence of adequate accountability mechanisms" for troops accused of violations.
Although Israel has publicly announced an investigation into events at the Gaza border, "an independent and impartial investigation has not yet taken place," they said.
The committee urged Israel to quickly initiate such a probe, and also to "counter the tide of racism and xenophobia in public discourse ... (and) to combat racist acts and manifestations of racist hate speech that particularly target Palestinians."