Ranks of nuns sang in Latin, accompanied by organ music, as Francis -- making the first papal visit to the mainly Buddhist nation -- delivered a homily urging compassion, opening his speech with "minglabar", Burmese for hello.
"I can see that the Church here is alive," he said of a Catholic community numbering around 700,000 -- a tiny fraction of the country's 51 million people.
Pope arrives in Myanmar on high-stakes visit
Earlier Francis smiled and waved as he moved through the estimated 150,000 faithful in his "popemobile". Many worshippers held Myanmar flags and wore colourful costume from the country's myriad ethnic groups.
"I never dreamed I would see him in my lifetime," said Meo, an 81-year-old from the Akha minority in Shan state. Like many others at the mass she is from one of Myanmar's conflict-riddled borderlands, where minority groups have long been marginalised by the Buddhist-dominated state.
"This is the most Catholics I have ever seen," added Gregory Than Zaw, 40, an ethnic Karen, who made the five-hour bus journey to Yangon with 90 people from his village.
The pope is set to hold a meeting with Buddhist leaders later Wednesday, on a visit that has been strong political as well as religious overtones.
Pope deliberately avoids using the term 'Rohingya' in key Myanmar speech
The pontiff arrived on Monday in a country on the defensive after an outcry from the international community over the plight of its Rohingya Muslim minority, who have been driven to Bangladesh in huge numbers.
He held private talks with both civilian leader Aung San Suu Kyi and the powerful army chief Min Aung Hlaing.
But Francis avoided mentioning the crisis -- or the Rohingya -- directly in a speech in the country's capital Naypyidaw on Tuesday, calling simply for "respect for rights and justice".
His caution so far on a four-day trip will bring relief to Myanmar's Catholic leaders, who had urged the pontiff not to wade into the treacherous issue for fear of sparking a backlash from Buddhist hardliners.
Even the mention of the name Rohingya is incendiary to many among the majority-Buddhist population, who deny the group are a distinct minority and insist on calling them "Bengalis".
Reactions to the pope's handling of the issue were mixed, with some Rohingya expressing disappointment that he did not draw direct attention to their suffering as he had done in several appeals from abroad.
But Kyaw Min, a former MP and prominent Rohingya activist, said he understood the pressures the pope was under and the pontiff had managed to speak about the group's plight indirectly.
Pope to meet Myanmar's leader Suu Kyi
"He said there were some people who have been bullied and need to get their rights ... this was about the Rohingya," Kyaw Min said.
A military crackdown has forced more than 620,000 Rohingya over the last three months to flee their homes in northern Rakhine state to one of the world's biggest refugee camps in neighbouring Bangladesh.
The army has justified the campaign as a proportionate retaliation for deadly attacks by hardline Rohingya militants in August.
But the UN and the US have labelled it ethnic cleansing and rights groups accuse the military of crimes against humanity, with refugees recounting consistent reports of murder, rape and arson.
On December 5 the UN's Human Rights Council will hold a special session to discuss the Rohingya crisis. There have been Catholics in Myanmar for over 500 years and they generally enjoy good relations with the Buddhist majority.
In the last three years, the Vatican has canonised Myanmar's first saint and named its first cardinal before full diplomatic ties were established in May this year, paving the way for the pontiff's visit.
"When we heard the sound of his words, we could tell they came from the heart... and that gives us peace," 47-year-old Yangon resident Ko Ko Lay said after the mass had finished.
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