He knew his voice was quavering. But U Kyaw Moe Tun, Myanmar’s top envoy at the United Nations, kept going. The military rulers who had overthrown Myanmar’s elected government and gunned down peaceful protesters were illegitimate, he said.

The words stumbled out, both a bit too high and a bit too low. “We will continue to fight,” he said, “for a government which is of the people, by the people, for the people.”

Mr. Kyaw Moe Tun, a 51-year-old diplomat in a somber suit and tie, raised his hand in the three-finger salute of defiance from the “Hunger Games” films, which has come to symbolize Myanmar’s millions-strong protest movement against the coup-makers. The United Nations General Assembly meeting in New York resounded with applause.

When Mr. Kyaw Moe Tun got home that night, Feb. 26, his family gathered around him. He had not told them what he had planned to do, he said. His 12-year-old daughter, like any preteen girl, had some feedback.

Dad, she said, you did the three-finger salute all wrong. Your fingers are supposed to be together, not apart. She said she was proud of him anyway, Mr. Kyaw Moe Tun recalled.

“The young generation, they know about democracy,” he said. “I also know about democracy, and I wanted to do something with maximum impact, to show how shocked I am, that in the modern world for a military to have a coup like this is not acceptable.”

When the generals seized full power in Myanmar on Feb. 1, curtailing a decade-long experiment with limited democratic reforms, they surely did not expect that it would ignite so much resistance among those responsible for the functioning of society: diplomats, teachers, doctors, railroad workers, bank tellers, power station employees, even police officers.

As young, unarmed protesters have poured onto the streets every day, defying bullets and arbitrary detention, others have sustained a civil disobedience movement, known in Myanmar as C.D.M., by refusing to work for the military overlords. Much of the state has stopped operating. Banks are closed, and government clinics are empty. Some train lines have been stilled.

And in the rarefied confines of Myanmar embassies around the world, diplomats are struggling with whether to represent a military that has locked up their elected leaders.

“I decided whatever I will do, I will resist,” Mr. Kyaw Moe Tun said. “I will never accept the military regime.”

Mr. Kyaw Moe Tun was not born a rebel. His father worked for a socialist party associated with the army chief who staged Myanmar’s first coup in 1962, ushering in nearly half a century of isolationist military rule.

In 1988, when other students at the University of Yangon joined mass pro-democracy demonstrations, he stayed off the streets of Myanmar’s biggest city.

“To be very frank, I did not take part in the protests,” Mr. Kyaw Moe Tun said. “My parents wanted me at home.”

One of his classmates in international relations was U Ko Ko Gyi, who would gain fame as a student leader of the 1988 movement. After the military crushed those protests, Mr. Ko Ko Gyi spent 17 years in prison.

“I’m surprised and so proud that U Kyaw Moe Tun actively joined the C.D.M.,” Mr. Ko Ko Gyi said in an interview.

After the bloody suppression of the 1988 protests, universities were forced to close. Mr. Kyaw Moe Tun was a third-year student without a degree or a way to make a decent living. Burma, as the country was then known, may have once been renowned for its polyglot cities, fertile rice paddies and leading universities, but decades of inept army rule had left the country to rot.

Like millions of people from Myanmar, Mr. Kyaw Moe Tun escaped overseas as a migrant worker. He assembled refrigerators and spray-painted fluorescent light covers in Malaysia, then joined a ship crew in Singapore.

That first trip abroad, in December 1988, astonished him. Landing at the Bangkok airport, he found the air-conditioning sumptuous, the television intoxicating. And there were lights everywhere, a feast of electricity. Yangon perpetually suffered blackouts, darkness descending after sundown; the sticky air was not conditioned.

After schools reopened in 1991, Mr. Kyaw Moe Tun came home. It was hard, he said, to describe to his parents just how much his homeland lagged most of the region. By the turn of the century, financial sanctions, imposed by Western nations on Myanmar for the junta’s appalling human rights record, were pulling the country even further behind.

After graduating from college, Mr. Kyaw Moe Tun rose steadily in the Ministry of Foreign Affairs, even as some of his former classmates suffered as political prisoners. He served as a third secretary at the embassy in Jakarta, Indonesia’s capital, then worked in New York and Singapore.

In 2011, the junta gingerly began to open up the country. Four years later, elections were held in which Daw Aung San Suu Kyi’sNational League for Democracy overwhelmed the military-backed party.

Ms. Aung San Suu Kyi, who had spent 15 years under house arrest, became foreign minister and the country’s de facto civilian leader. The military still controlled much of government, Parliament and the economy, but Myanmar was no longer isolated in tropical totalitarianism.

In 2018, Mr. Kyaw Moe Tun was dispatched to Geneva as ambassador and representative to the United Nations offices there. While the halting political transition unfolding in Myanmar had won star-struck admirers like President Obama, who visited twice, the reality of the military’s reflexive brutality intruded with the ethnic cleansing of Rohingya Muslims, a campaign that intensified in 2017.

Rather than condemn the systematic executions, rapes and village burnings, Ms. Aung San Suu Kyi, a Nobel Peace laureate, defended the generals. There was little outcry in Myanmar over the brutal persecution of ethnic minorities. Ms. Aung San Suu Kyi defended the military at The Hague, where Myanmar was accused of genocide against the Rohingya. Myanmar’s diplomats, including Mr. Kyaw Moe Tun, fell in line, earning the country international scorn.

Last October, Mr. Kyaw Moe Tun presented his credentials as Myanmar’s permanent representative to the United Nations. Back home, rumors of a coup simmered before the November elections, which the National League for Democracy won by a landslide. The military cried foul, and talk of a putsch escalated.

On Feb. 1, the military, led by Senior Gen. Min Aung Hlaing, arrested the nation’s civilian leadership, later charging Ms. Aung San Suu Kyi and the nation’s president with obscure crimes. Dozens of foreign ministry officials were detained after participating in the civil disobedience movement.

In overseas missions, envoys agonized over what to do. Daw Chaw Kalyar, now at the Myanmar Embassy in Berlin, recalled how as a high school student in 1988 she had marched in the mass protests before security forces killed hundreds or possibly thousands of people. Since the Feb. 1 coup, more than 60 people have been shot dead by the security forces.

“The military don’t regard the people of Myanmar as people. They just shoot everyone,” Ms. Chaw Kalyar said. “That feeling, in 1988, at that time, we lost and we had to give in. I cannot believe it’s happening again.”

After Mr. Kyaw Moe Tun’s defiant salute at the United Nations and more lethal attacks on unarmed protesters, Ms. Chaw Kalyar and another envoy in Berlin joined the civil disobedience movement. Diplomats stationed in Los Angeles, Geneva, Tel Aviv and Washington are participating, too.

“We risk everything but it’s not comparable to those on the streets who risk everything,” Ms. Chaw Kalyar said. “We decided to join the C.D.M. because we have to fight back.”

In New York, Mr. Kyaw Moe Tun was fired by Myanmar’s military rulers and accused of high treason. But he refused to go, and the diplomat who was chosen to replace him quit. The United Nations itself has declined to recognize Mr. Kyaw Moe Tun’s dismissal. For now, at least, he is staying.

“I’m a civil servant and I take instructions from the government, but the military illegally took state power,” Mr. Kyaw Moe Tun said. “This is the time to express our true colors, our real desires. It’s our duty to Myanmar people.”