Troops opened fire on pro-democracy protesters on Wednesday, killing at least 15 people and wounding dozens, activists and media said.
The dead include 46 children, according to the Assistance Association for Political Prisoners activist group, yet protesters are vowing to continue daily "guerrilla rallies."
At least 510 civilians had been killed in nearly two months of efforts to stop protests, an advocacy group said, while the death toll from Saturday alone had risen to 141.
Protests were held in Yangon and Mandalay, the country's two biggest cities, as well as elsewhere. Some of the demonstrations were again met with police force.
Domestic media reported two protesters were killed in police firing in the Tharketa district of Myanmar's commercial capital Yangon overnight.
South Korea also said it would suspend defense exchanges and reconsider development aid to the Southeast Asian nation because of the military's harsh crackdown on the protests.
Six people were killed in the central town of Myaing when forces fired on a protest, according to a witness, while one person was killed in Yangon, Myanmar's biggest city.
In what has become a daily occurrence, protest marches were held Tuesday in cities and towns across the country, according to local news reports and social media.
"It's horrific, it's a massacre. No words can describe the situation and our feelings," youth activist Thinzar Shunlei Yi told Reuters via a messaging app.
At least five people were injured by rubber bullets and had to be carried away in ambulances, according to an Associated Press journalist who witnessed the violence.
A small crowd began gathering outside the central bank early on Tuesday to press staff there to join a civil disobedience movement.
Large demonstrations were held in the major cities of Yangon, Mandalay and the capital, Naypyitaw, as well as in far-flung areas dominated by ethnic minorities.
"My reaction in the beginning was disbelief,” said Khin Yin, who moved from Myanmar to Canada 23 years ago.