The killing of reportedly at least 114 unarmed civilians - including a five-year-old boy and a 13-year-old girl - by Myanmar's security forces in the single deadliest day since the military coup on 1 February has drawn strong condemnation from world leaders.
The shootings came as people took to the streets again on Saturday to protest against the military takeover, in defiance of a warning that they could be shot "in the head and back", as the junta celebrated Armed Forces Day.
Demonstrators had referred to the public holiday by its original name, Resistance Day, to mark the beginning of a revolt against Japanese occupation in the Second World War.
This year's event was seen as a flashpoint, with protesters threatening to bolster their public opposition to the coup with more and bigger demonstrations.
In response to the latest killings, UN special rapporteur Tom Andrews said it was time for the world to take action - if not through the UN Security Council then through an international emergency summit.
"Words of condemnation or concern are frankly ringing hollow to the people of Myanmar while the military junta commits mass murder against them," Mr Andrews said in a statement.
"The people of Myanmar need the world's support. Words are not enough. It is past time for robust, coordinated action."
US ambassador to Myanmar Thomas Vajda said on social media: "On Myanmar's Armed Forced Day, security forces are murdering unarmed civilians, including children, the very people they swore to protect.
"This bloodshed is horrifying. These are not the actions of a professional military or police force."
The EU delegation to Myanmar said the killings would "forever stay engraved as a day of terror and dishonour", while Britain's Foreign Secretary Dominic Raab said the violence marked "a new low".
Dr Sasa, the disbanded democratic parliament's special envoy to the UN, said: "Today is a day of shame for the armed forces. The military must stop killing its own people."
Calling on the international community to "act quickly before it's too late" and bring in coordinated, targeted sanctions, he told Sky News that the military must release democratically elected leader Aung San Suu Kyi and come to the negotiating table.
Without action, he said, Myanmar would see the "greatest civil war that we've never seen before".
Meanwhile, top military officers from 12 countries are set to condemn the deadly use of force by Myanmar's security forces and say the country's military has lost credibility with its people.
The draft statement, signed by 12 chiefs of defence from the US, UK, Australia, Canada, Denmark, Germany, Greece, Italy, Japan, Netherlands, New Zealand and South Korea, is will say: "As Chiefs of Defence, we condemn the use of lethal force against unarmed people by the Myanmar Armed Forces and associated security services."
The shootings came as a rebel leader told Sky News that Myanmar's armed ethnic groups will not stand by and do nothing if the military continued to kill demonstrators.
After leading a military parade in the capital Naypyitaw, the junta's leader General Min Aung Hlaing repeated that his regime would hold "a free and fair election" and hand over power afterwards, but gave no timetable.
In a live broadcast on state TV, he said: "The army seeks to join hands with the entire nation to safeguard democracy.
"Violent acts that affect stability and security in order to make demands are inappropriate."
The number of people killed in the unrest since the overthrow of Aung San Suu Kyi's elected government now stands at around 440.
However, monitors have cautioned this only includes verified cases, with the actual number of casualties "likely much higher".
The Myanmar Now news portal said at least 114 people were killed across the country in the crackdown on Saturday.
It added that around 90 people were killed in Yangon, and dozens more died - one of them a five-year-old boy and a 13-year-old girl - in Mandalay.