Israeli forces have opened fire on Palestinian protesters rallying against the opening of the US embassy in Jerusalem.
Palestinian officials say at least 52 have been killed at the border between Gaza and Israel and 2400 injured on Monday (local time) - many by live bullets.
It marks the highest Palestinian single-day death toll since the 2014 Gaza war.
Tens of thousands gathered at the frontier, 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.
The dead included a 14-year-old boy, a medic and a man in a wheelchair who had been pictured on social media using a slingshot.
The Israeli military identified three of those killed as armed militants whom it said tried to place explosives near the fence in the southern Gaza Strip.
"The IDF (Israel Defence Forces) will act forcefully against any terrorist activity and will operate to prevent attacks against Israelis," the military said in a statement.
"Today is the big day when we will cross the fence and tell Israel and the world we will not accept being occupied forever," Gaza science teacher Ali, who declined to give his last name, told Reuters.
"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.
Israeli Prime Minister Benjamin Netanyahu echoed the sentiment, describing the opening of the US Embassy as an historic moment.
"What a glorious day. Remember this moment. This is history.
"President Trump, by recognising history, you have made history."
The killings have drawn international criticism, but the US has echoed Israel in accusing Gaza's ruling Hamas movement of instigating violence, an allegation it denies.
Jason Greenblatt, Mr Trump's Middle East peace envoy, said on Twitter that "taking the long-overdue step of moving our Embassy is not a departure from our strong commitment to facilitate a lasting peace deal. Rather, it is a necessary condition for it."
But 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 Mr Trump's shift from previous administrations' preference for keeping the US Embassy in Tel Aviv pending progress in peace efforts.
But CNN's Oren Liebermann says Israel leaders will still be feeling positive about recent developments.
"President Donald Trump withdrew from the Iran nuclear deal as Prime Minister Benjamin Netanyahu was pushing him to do.
"That leads into this week - the US opening of the embassy."
Reuters / Newshub.