Palestinian officials say that Israeli forces have shot dead at least 52 protesters on Gaza's border with Israel
It is the deadliest day since Gazans began protests dubbed the "Great March of Return" six weeks ago
The protests coincided with the controversial opening of the US embassy in Jerusalem
The decision has infuriated Palestinians, who see it as clear US support for Israeli rule over all of the city
UN Secretary General António Guterres expresses concern about the reported death toll at the protests in Gaza, the Associated Press reports.
"I'm particularly worried about the news coming from Gaza with the high number of people killed," he tells reporters in Vienna.
US Treasury Secretary Steve Mnuchin and Mr Trump's daughter, Ivanka, unveiled a plaque on the wall of the building that will serve as the temporary home of the US embassy.
"On behalf of the 45th president of the Unites States of America, we welcome you officially and for the first time to the embassy of the United States here in Jerusalem, the capital of Israel," Ms Trump said.
President Trump did not travel to Jerusalem for the opening ceremony, but a video message from him was just played on a big screen there.
He said: "Exactly 70 years ago today the United States under Harry Truman became the first nation to recognise the state of Israel. Today we open the US embassy in Jerusalem. It's been a long time coming."
He added: "Today Jerusalem is the seat of Israel's government. It is the home of the Israeli legislature and the Israeli supreme court and Israel's prime minister and president.
"Israel is a sovereign nation with the right like every other sovereign nation to determine its own capital. Yet for many years we failed to acknowledge the obvious. The plain reality that Jerusalem is Israel's capital.
Mr Trump said he had accomplished the relocation "many many years ahead of schedule", adding that the US would "always be a great friend to Israel and a partner in the cause of freedom and peace".
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Palestinian health officials say at least 41 people have now been shot dead and 1,800 others injured by Israeli troops amid clashes along the Gaza-Israel border fence.
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The ceremony in Jerusalem began with the US ambassador to Israel, David Friedman, officially declaring the new embassy open.
"Today, we open the United States embassy in Jerusalem, Israel," he said. "Again the US leads the way as the first nation to do so."
He credited Mr Trump's "courage and moral clarity" for the embassy move, adding: "We owe an enormous and eternal debt of gratitude."
Mustafa Barghouti, an independent Palestinian MP, tells the BBC that the US embassy move will lead to a rise in tensions and bloodshed:
Quote Message"The Israelis are thinking that they've got a green light from the Americans to do whatever they want. What we see today is a real massacre. So far, the Israelis have responded to our peaceful marches with lethal weapons. It's a very dangerous situation and this is going to really cause much more victims on the Palestinian side."
At least 37 Palestinian protesters have been killed so far at Monday's protests, according to Gaza health officials. No Israeli fatalities have been reported.
Monday has been the deadliest day since the the start of a series of protests on the border by Palestinians that has been dubbed the "Great March of Return".
Palestinian health officials say at least 37 Palestinians have died so far today, taking the total death toll since protests began on 30 March to at least 90.
Israel says its troops have opened fire to stop infiltrators crossing the border fence.
Image source, ReutersProtester scramble for cover near the Gaza border
Jared Kusher, Donald Trump's son-in-law and aide, will defend the controversial decision to move the US embassy to Jerusalem when he speaks at the opening ceremony.
Mr Kushner, a former real-estate broker, was controversially charged by Mr Trump with shepherding a new Middle East peace process despite his lack of diplomatic experience.
According to excerpts published by Reuters news agency, Kushner will say:
Quote Message"While presidents before him have backed down from their pledge to move the American Embassy once they were in office, this president delivered. Because when President Trump makes a promise, he keeps it."
You can watch the proceedings live below:
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Israeli jets dropped leaflets on Gaza on Monday morning, ahead of the protests.
"You are taking part in violent riots which jeopardise your lives," the leaflets said.
They warned that the Israeli military would "act against any attempt to damage the security fence or harm IDF [Israel Defense Forces] soldiers or Israeli civilians".
Palestinians were urged not to let the Islamist militant group Hamas, which dominates the Gaza Strip, use them as "puppets".
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Why Jerusalem matters
This holy and disputed city is at the very heart of the Israeli-Palestinian conflict.
Israel regards Jerusalem as its "eternal and undivided" capital, while the Palestinians claim East Jerusalem - occupied by Israel in the 1967 Middle East war - as the capital of a future state.
Israeli sovereignty over Jerusalem has never been recognised internationally, and according to the 1993 Israel-Palestinian peace accords, the final status of Jerusalem is meant to be discussed in the latter stages of peace talks.
Since 1967, Israel has built a dozen settlements, home to about 200,000 Jews, in East Jerusalem. These are considered illegal under international law, though Israel disputes this.
Meanwhile, dignitaries and guests are gathered in Jerusalem for the inauguration of the new US embassy. The ceremony is due to start at 16:00 local time (13:00 GMT).
Among the guests are Israeli Prime Minister Benjamin Netanyahu, President Trump's daughter Ivanka, her husband Jared Kushner, and US Treasury Secretary Steven Mnuchin.
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The newly renamed US Embassy Jerusalem Twitter account is livestreaming images from ceremony - a stark contrast to the violent scenes from Gaza.
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The Israeli military says the Palestinian protests are being used as cover for acts of terrorism.
A spokesperson for the Israel Defense Forces (IDF) said three people had attempted to place an explosive device next to the Israel-Gaza border fence earlier on Monday.
Israeli troops fired at the assailants, killing them, the spokesperson added.
The IDF estimated that more than 35,000 people were protesting along the frontier. In a statement, it said the Israeli Air Force had targeted Hamas military posts near the Jabalia area after IDF troops were fired on from the northern Gaza Strip.
No soldiers were injured, the IDF said.
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Gaza health officials said at least 37 Palestinians had been killed and 1,300 others injured in Monday's violence.
Palestinians killed in Gaza-Israel border clashes
On Twitter, the US president hails the embassy relocation as a “great day for Israel", plugging the Fox TV network but neglecting to mention the deadly violence in Gaza.
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Palestinian health officials say at least 28 Palestinians have been killed and 1,000 others injured by Israeli forces during protests along the border between the Gaza Strip and Israel.
The protests began hours before a ceremony to mark a controversial decision by US President Donald Trump to relocate the US embassy in Israel from Tel Aviv to Jerusalem.
Read our full news story below and stay on this page for live updates.
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