The Chief of the Defence Staff, General Sir Nick Carter, has signed a joint letter condemning the actions of the Myanmar military.
The statement, signed by defence chiefs from 12 countries, including the US, Australia, Canada, Germany and Greece, said a professional military should protect, rather than harm, the people it serves.
Myanmar security forces reportedly killed more than 114 people on Saturday as they continued to crack down on protests calling for a return to democracy.
It was the bloodiest day since last month's military coup, which ousted Aung San Suu Kyi's elected government.
The letter added: "As Chiefs of Defence, we condemn the use of lethal force against unarmed people by the Myanmar Armed Forces and associated security services.
"We urge the Myanmar Armed Forces to cease violence and work to restore respect and credibility with the people of Myanmar that it has lost through its actions."
The coup has reversed years of progress towards democracy after five decades of military rule.
It has again made Myanmar the focus of international scrutiny as security forces have repeatedly fired into crowds of protesters.
At least 459 people have been killed since the takeover on 1 February, according to the Assistance Association for Political Prisoners (AAPP).
The junta has accused some of the demonstrators of perpetrating the violence because of their sporadic use of Molotov cocktails and has said its use of force has been justified to stop what it has called rioting.
On Saturday, UN Secretary-General Antonio Guterres said on Twitter that he was "deeply shocked by the killing of dozens of civilians, including children and young people by security forces".
"The continuing military crackdown is unacceptable and demands a firm, unified and resolute international response," he added.
US Secretary of State Antony Blinken said in a tweet that his country is "horrified by the bloodshed perpetrated by Burmese security forces, showing that the junta will sacrifice the lives of the people to serve the few".
Defence chiefs from Italy, Japan, Denmark, the Netherlands, New Zealand and South Korea have also added their names to the joint letter of condemnation.
Cover image: Chief of the Defence Staff, General Sir Nick Carter (Picture: PA).