Security forces opened fire in an area of Myanmar's largest city, Yangon, with protesters using sticks and knives.
The military declared martial law in the area after Chinese businesses were attacked. Protesters believe China is giving support to the military but it is unclear who was behind the attacks.
Myanmar has been gripped by protests since the military coup on 1 February.
Military rulers have detained Aung San Suu Kyi, the country's civilian leader and head of the National League for Democracy (NLD) party.
The NLD won a landslide in last year's election but the military alleged there had been widespread fraud.
Some of the ousted MPs have refused to accept last month's coup and have gone into hiding.
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In his first public address, their leader Mahn Win Khaing Than urged protesters to defend themselves against the military crackdown during what he called a "revolution".
"This is the darkest moment of the nation and the moment that the dawn is close," he said, adding: "The uprising must win."
At least 21 people were reportedly killed in Yangon on Sunday. Further deaths and injuries were reported elsewhere in the country. The Assistance Association for Political Prisoners (AAPP) monitoring group said the day's death toll was at least 38.
Medical workers said the number of people killed in the Yangon area of Hlaing Tharyar was likely to rise, with dozens suffering gunshot wounds.
What happened in Hlaing Tharyar?
The junta has declared martial law in Hlaing Tharyar and neighbouring Shwepyitha after China said Chinese factories in the area had been targeted and demanded protection.
Beijing said people armed with iron bars, axes and petrol had set alight and damaged 10 Chinese facilities - mostly clothing production or storage factories - in Yangon. A Chinese hotel was also attacked.
On its Facebook page the Chinese embassy said some "factories were looted and destroyed and many Chinese staff were injured and trapped".
The embassy urged Myanmar to "take further effective measures to stop all acts of violence, punish the perpetrators in accordance with the law and ensure the safety of life and property of Chinese companies and personnel in Myanmar".
The military-owned Myawaddy Media reported that firefighters had been hindered in their response to the blazes by people blocking their routes.
Gunshots were heard throughout the day and military trucks were seen in the streets.
Demonstrators barricaded themselves in with sandbags, car tyres and barbed wire when security forces opened fire. Using makeshift shields, some were seen inching forwards to retrieve the injured.
One officer posted on social media that police were planning to use heavy weaponry.
"I will not have mercy on Hlaing Tharyar and they will fight back seriously too because there are all kinds of characters there," the officer said in the subsequently deleted TikTok post.
"Three died in front of me while I was giving treatment. I'm sending another two to the hospital. That's all I can say at this moment," one medic told AFP.
Several other deaths at the hands of the military were reported in other parts of Myanmar, including a young man shot dead by security forces in the northern jade-producing city of Hpakant, and a man and a woman killed in Bago to the north of Yangon.
Meanwhile, state TV said one police officer had been killed. Three more were injured by protesters throwing rocks and using catapults in the Bago region, MRTV said.
In total, more than 120 protesters have reportedly been killed during the crackdown, according to the AAPP monitoring group.
Later in the evening, hundreds of people sat with lit candles raised at the main junction on Hledan road in central Yangon.
What did Mahn Win Khaing Than say?
NLD MPs who managed to escape arrest after the coup formed a new group, the CRPH, or Committee for Representing the Union Parliament.
Mahn Win Khaing Than was appointed the CRPH's acting head and the group is seeking international recognition as Myanmar's rightful government.
In a speech on Facebook, Mahn Win Khaing Than said: "This is the time for our citizens to test their resistance against the dark moments.
"In order to form a federal democracy, which all ethnic brothers who have been suffering various kinds of oppressions from the dictatorship for decades really desired, this revolution is the chance for us to put our efforts together.
"Despite our differences in the past, this is the time we must grip our hands together to end the dictatorship for good. "
The military considers the CRPH to be an illegal group, warning that anyone co-operating with them will face treason charges.
What's the background?
Independent international observers have disputed the military's claim of the fraudulent election held in November 2020, saying no irregularities were observed.
Last week, the military accused Ms Suu Kyi of illegally accepting $600,000 (£430,000) and 11kg of gold. No evidence was provided and an NLD lawmaker denied the allegation.
Ms Suu Kyi has been held for the past five weeks at an undisclosed location and faces several other charges including causing "fear and alarm", illegally possessing radio equipment, and breaking Covid-19 restrictions.
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Since the coup the military has used violent force to try to quell protests, leaving dozens dead and prompting widespread international condemnation.
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The US has announced sanctions on coup leaders, while steps are also being taken to block access by the military to $1bn of government funds held in the US.
The military has dismissed criticism of its actions, instead of blaming Ms. Suu Kyi for the violence.
- Myanmar became independent from Britain in 1948. For much of its modern history, it has been under military rule
- Restrictions began loosening from 2010 onwards, leading to free elections in 2015 and the installation of a government led by veteran opposition leader Aung San Suu Kyi the following year
- In 2017, Myanmar's army responded to attacks on police by Rohingya militants with a deadly crackdown, driving more than half a million Rohingya Muslims across the border into Bangladesh in what the UN later called a "textbook example of ethnic cleansing" Instagram
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