YANGON — Security forces on Sunday intensified their crackdown on protests in Myanmar during the bloodiest day of demonstrations since the start of the military coup a month ago, killing at least 18 and using lethal force against protesters in the main city of Yangon for the first time.

The clashes — with tear gas, stun grenades, rubber bullets and live rounds — turned Yangon and cities across the country into battlegrounds as the military moved to crush resistance to its deeply resented seizure of power.

The United Nations’ human rights office said at least 18 people died and 30 others were wounded in cities including Yangon, Dawei, Mandalay and Bago. The deaths, the office said, occurred “as a result of live ammunition fired into crowds.”

“Use of lethal force against non-violent demonstrators is never justifiable under international human rights norms,” the office said.

A protester who was in the Yangon neighborhood of Hledan when police opened fire said they gave only one short whistle blast as a warning and immediately began shooting.

“First they shot with real bullets, then tear gas. Later they used rubber bullets,” said the protester, who identified himself only by part of his name, Yan, out of fear of retaliation from security forces.

He said he saw a man shot in the head who he believed had died, and six others who had suffered gunshot wounds.

“Now people are regrouping and protesting again,” Yan said. He said the violence has made protesters “angrier,” not scared.

U.S. national security adviser Jake Sullivan said the country is “alarmed” by security forces’ violence against “peaceful protesters.” He referred to Myanmar as Burma, a former name.

“The killings today represent an escalation,” Sullivan said in a statement. “The United States stands in solidarity with the people of Burma, who continue to bravely voice their aspirations for democracy, rule of law, and respect for human rights.”

He said the United States was “preparing additional actions to impose further costs on those responsible for this latest outbreak of violence and the recent coup,” with details available in the coming days.

Mass protests began soon after the military arrested State Counselor Aung San Suu Kyi and seized power Feb. 1. Her National League for Democracy party won reelection in a landslide in November, but military leaders alleged fraud and refused to recognize the results.

Sunday’s violence marks a significant escalation, particularly in Yangon, which had largely avoided a severe crackdown, even as protesters were killed in other parts of the country. One man was shot dead by police in Yangon’s Shwe Pyi Thar township on Feb. 21 during a civilian neighborhood watch patrol, but Sunday’s violence was the first time people died during protests.

Confrontations with police in downtown Yangon began with a crowd forming around a prison truck transporting a group of students who had been arrested. The protesters advanced toward a police line, which then charged, sending people scurrying down different side streets.

Police then began firing stun grenades as protesters took shelter in homes and shops. At one point, a middle-aged protester walked back into the street, facing the police alone. “Shoot me, don’t shoot the young people!” he shouted.

Later, a group of teachers and lawyers assembled outside the Kyauktada police station, where the students were being held. “There are 16 students in the station and in the car maybe more than 17,” said a foreign-language teacher. “Almost all of our students have been arrested.”

The teacher, who spoke on the condition of anonymity for fear of retaliation, explained that lawyers were trying to negotiate with the police not to send the students straight to Insein Prison, an increasingly common practice as arrests mount.

The Assistance Association for Political Prisoners said more than 850 people have been arrested, charged or sentenced since the start of the coup.

Meanwhile, the city’s Myaynigone neighborhood, a hip enclave of bars and cafes, has been transformed into a battleground with barriers erected on major roads to slow down police. There are also spikes and slicks of diesel. As reporters entered the area, lookouts called down from balconies to warn them about the obstacles.

One of the barricades was manned by a group of software engineers, including one who identified himself as Thiha. He asked that his full name not be used out of concern for authorities. He said police had fired tear gas, rubber bullets and stun grenades at them earlier in the day.

“I was crying a little bit, but [the gas] didn’t affect me too badly because I was far away,” he said. He said he and his friends were protesting “for freedom” and for the future of his career.

“Everything we worked for is blocked without a VPN,” he said. Since seizing power, the junta has imposed strict controls on the Internet.

Deeper in the neighborhood, protesters in hard hats, some armed with bats and steel rods, milled around, singing and chanting. Some handed out boxes of food, with phrases like “the revolution must succeed” scrawled on the plastic foam.

Protesters later said the police advanced on their area again and dispersed people.