The United States has forfeited its authority to broker a solution to the Israel-Palestine conflict and has become "part of the problem, not the solution", Turkey's president has said.
Recep Tayip Erdogan's comments were among the most strongly worded of a series of diplomatic protests as government around the world condemned Donald Trump's decision to move the US embassy to Jerusalem.
The United States formally opened an embassy in Jerusalem on Monday afternoon, after Mr Trump recognized Jerusalem as Israel's capital last year.
Palestinian health officials said 52 people were killed and more 1200 injured by Israeli fire during protests against the opening of a US embassy in Jerusalem.
Mr Erdogan, who is in London on a three day visit for talks with Theresa May, said the move could "ignite an even greater fire between communities."
"With its latest step America has chosen to be a part of the problem, not a solution, and lost its mediator role in the Middle East peace process", he said in a speech at Chatham House
"We are rejecting once again this decision which violates international law and which is against UN resolutions," he said.
"The international community must do its part as soon as possible and take swift action to put an end to Israel's increasing aggression," he went on.
In a separate statement, Mrs May called for "restraint" and criticized the move, but avoided Mr Erdogan's direct condemnation.
"We are concerned by the reports of violence and loss of life in Gaza. We urge calm and restraint to avoid actions destructive to peace efforts," a Number 10 spokesman said.
"The prime minister had made her views clear in December that we disagreed with the decision (to move the US embassy). We believe it's unhelpful in terms of prospects for peace in the region."
Extremely saddened by loss of life in Gaza today. Concerned peaceful protests are being exploited by extremist elements. Urge restraint in use of live fire. Violence is destructive to peace efforts. UK remains committed to a two-state solution with Jerusalem as a shared capital.— Alistair Burt (@AlistairBurtUK) May 14, 2018
Alistair Burt, the Foreign Office minister for the Middle East, tweeted: "Extremely saddened by loss of life in Gaza today. Concerned peaceful protests are being exploited by extremist elements. Urge restraint in use of live fire. Violence is destructive to peace efforts. UK remains committed to a two-state solution with Jerusalem as a shared capital."
But other governments took a much stronger line.
Jean-Yves Le Drian, the French foreign minister, called the embassy move a violation of International law and of the UN Security Council resolutions.
Referring to the day's violence, he said: "France calls on all actors to show responsibility to prevent a news escalation and calls on the Israeli authorities to exercise discernment and restraint in the use of force that must be strictly proportionate," he said.
Mikhail Bogdanov, the Russian deputy foreign minister responsible for Middle Eastern affairs, called the move "short sighted" and said the US was to blame for the "sharp escalation" of violence in Gaza on Monday.
The Palestinian Authority, which controls the West Bank but not the Gaza strip, called the killings a "massacre" and demanded an emergency meeting of the UN Security council
"If a similar massacre (by an occupying power) happens in any other nation, it would trigger a massive global outrage. Palestine should not be an exception" said Riyad Mansour, the Palestinian ambassador to the UN.
Saudi Arabia and Kuwait have also urged a UN emergency meeting.
Zeid Ra'ad al Hussein, the UN high commissioner for human rights, called the violence in Gaza "shocking" and demanded that Israel halt the use of live rounds.
“Shocking killing of dozens, injury of hundreds by Israeli live fire in #Gaza must stop now. The right to life must be respected. Those responsible for outrageous human rights violations must be held to account. The int'l community needs to ensure justice for victims,” he said in a Tweet put out by his office.
EU foreign policy chief Federica Mogherini urged "utmost restraint", saying "dozens of Palestinians, including children, have been killed and hundreds injured from Israeli fire today, during ongoing mass protests near the Gaza fence. We expect all to act with utmost restraint to avoid further loss of life"
Iran, which is locked in a diplomatic and military confrontation with Israel over its presence in Syria, called it "a day of great shame".
"Israeli regime massacres countless Palestinians in cold blood as they protest in world's largest open air prison. Meanwhile, Trump celebrates move of U.S. illegal embassy and his Arab collaborators move to divert attention," said Javad Zarif, the Iranian foreign minister.