Pictures and footage from Wednesday show bodies lying in the street surrounded by pools of blood as protesters take cover.
The United Nations said the death toll had risen to 50 since the coup, despite activists calling that number higher.
“Today was the bloodiest day since the coup,” said Special Representative Christine Schraner Burgener on Wednesday at a briefing. About 1,200 people have been arrested and many relatives are unsure where they are being held, she added.
“Every available tool is now needed to stop this situation,” said Burgener. “We need unity in the international community, so it is up to the Member States to take the right action.”
CNN has emailed the ruling military regime but has not yet received a response.
The demonstrators have been calling for the release of democratically elected officials – including the head of state Aung San Suu Kyi – who are in custody for weeks. Suu Kyi’s National League for Democracy Party won a landslide victory in November’s elections. Military leaders accuse election fraud but have provided no evidence to support their claims.
Burgener said in talks with the military that she had warned that the UN Security Council and member states were likely to take strong action. “The answer was, ‘We are used to sanctions and have survived those sanctions in the past,'” she said.
“When I also warned that they would be isolated, the answer was, ‘We have to learn to run with few friends’.”
Security forces – including members of the military’s light infantry divisions, long documented implications for human rights abuses in conflict areas across the country – escalated their deadly crackdown on peaceful protesters this week.
“Today the country is like Tiananmen Square in most of its major cities,” said the Archbishop of Yangon, Cardinal Charles Maung Bo, on Twitter.
In one case, security forces in Myanmar were caught on camera hitting rescue workers with the butt of their guns and batons and kicking them in the head, according to activist group Assistance Association for Political Prisoners (AAPP).
The AAPP released the video on Wednesday and said in a statement that it was from northern Okkalapa, Yangon. The video gives an insight into the brutal methods used by the security forces.
In the footage, three charities are asked to get out of their ambulance at gunpoint and then kneel on the floor with their hands behind their heads.
Two uniformed police officers hit the men in the head with their guns and batons and kick them too. A few moments later, a group of police officers with signs and members of the military join in and violently beat the charity workers.
“The military is treating peaceful protesters in Yangon as a war zone. The military is creating terror again,” AAPP said.
CNN doesn’t know why the charities were stopped by the security forces.
The AAPP said live ammunition was used against protesters in at least seven cities on Wednesday.
Among those killed was a 19-year-old girl in the second largest city, Mandalay. Her picture flooded social media sites, showing her in a t-shirt that read “Everything will be fine”. Reuters reported that she was shot in the head by security forces.
In Myanmar’s largest city, Yangon, witnesses said Reuters had killed at least eight people when security forces opened fire with automatic weapons in the early evening.
“I’ve heard so much non-stop shooting. I lay down on the ground, they shot a lot and I saw two people killed on the spot,” 23-year-old protester Kaung Pyae Sone Tun told Reuters.
Another major toll was in downtown Monywa, where six people were killed, the Monywa Gazette reported. Others were killed in various locations, including Mandalay, the northern city of Hpakant and the central city of Myingyan, according to Reuters.
Rights group Fortify Rights said Thursday that “the similar use of excessive and deadly force by security forces in cities across the country demonstrates inter-unit coordination and an overarching national strategy.”
“This is not a non-lethal tactic to disperse protesters. This is an attack on peaceful protesters across the country,” said John Quinley, senior human rights specialist at Fortify Rights. “And these are not crowd control techniques, but an attack on civilians and people protesting the military takeover.”
The right-wing group said Wednesday photos and videos show soldiers holding automatic weapons, long-range sniper rifles and other firearms.
World leaders have called for the restoration of the elected leaders in Myanmar.
“The use of lethal force against peaceful demonstrators and arbitrary arrests is unacceptable,” said a statement by UN Secretary General Antonio Guterres’ spokesman on February 28, calling on the international community to “send a clear signal to the military that she must respect the law. ” Will of the people of Myanmar as expressed through the elections. “
A speech by Myanmar’s UN Ambassador, Kyaw Moe Tun, sparked rare applause last week after he said he represents the country’s civilian government and called on the international community to use “all necessary means” to end the coup.
A Deputy Ambassador to Myanmar, U Tin Maung Naing, resigned on Wednesday after the military named him to succeed Kyaw Moe Tun.
The US State Department condemned the violence, saying Washington is reviewing policy options to respond to recent escalations.
“We are appalled and outraged that the terrible violence against the people of Burma is being carried out because of their peaceful calls for the restoration of civil governance. We call on all countries to speak with one voice to oppose the brutal violence of the Burmese military against it to condemn and condemn their own people. ” Promote accountability for the actions of the military that have killed so many people in Burma, “State Department spokesman Ned Price said at a press conference.
Pope Francis also called for an end to the violence on Wednesday.
“I also appeal to the international community to act so that the aspirations of the people of Myanmar are not suppressed by violence. That the young people in this beloved country have the opportunity to hope in a future of hatred and injustice through meeting and meetings to be replaced. ” Reconciliation, “he said during his weekly audience.
CNN’s Pauline Lockwood, Akanksha Sharma, Mitchell McCluskey and Jennifer Deaton contributed to this report.