More than 90 deaths, including children, were confirmed by the Assistance Association for Political Prisoners (AAPP) monitoring group.
“They are killing us like birds or chickens, even in our homes,” resident Thu Ya Zaw told Reuters news agency in the central town of Myingyan.
“We will keep protesting regardless.”
The lethal crackdown came as protesters defied warnings and took to the streets on the annual Armed Forces Day.
US, UK and EU officials condemned the violence, with British Foreign Secretary Dominic Raab calling it a “new low”.
The AAPP said the death toll was continuing to rise.
The latest violence took the number killed in the suppression of protests in Myanmar since the 1 February coup to more than 400.
The military seized control of the South East Asian country after an election which Aung San Suu Kyi’s National League for Democracy (NLD) party won by a landslide.
What is happening on the streets?
Protesters gathered across Myanmar, also known as Burma, on Saturday.
State TV aired an announcement the previous evening saying people “should learn from the tragedy of earlier ugly deaths that you can be in danger of getting shot to the head and back”.
Security forces were out in strength trying to prevent rallies.
Images shared on social media showed people with gunshot wounds and families mourning.
The director of the Burma Human Rights Network in UK told the BBC the military had shown it had “no limits, no principles”.
“It’s a massacre, it’s not a crackdown anymore,” Kyaw Win said.
Violent crackdowns using live ammunition were reported in more than 40 locations across the country.
Local news site Myanmar Now put the death toll at 114, while the United Nations said it was receiving reports of “scores killed” and hundreds more injured.
The AAP said among the fatalities was a 13-year-old girl who was shot dead inside her home.
In the main city Yangon, gunshots were fired at the US cultural centre on Saturday. The US embassy said those shots caused no injuries.
Witnesses and sources told BBC Burmese of protester deaths in the cities and townships of Magway, Mogok, Kyaukpadaung and Mayangone.
Deaths were also reported in Yangon and on the streets of the second-largest city Mandalay, where protesters carried the flag of the NLD and gave their now traditional anti-authoritarian three-finger salute.
Meanwhile, an ethnic armed group in eastern Myanmar said military jets had targeted territory it controls. The strike was launched hours after the group, called Karen National Union, said it had overrun an army post near the Thai border.
It came amid rising tensions between the group and the military after years of relative peace.
Children among the dead and injured
Moe Myint, BBC Burmese
A one-year old girl was hit in the eye with a rubber bullet as she played on the pavement near her home in a suburb of Yangon.
A five-year-old boy in Mandalay is fighting for his life after being shot in the head by security forces.
Across the country, children are amongst the injured and the dead in the bloodiest day since the coup on the 1 February.
Fourteen-year-old Pan Ei Phyu’s mother says she rushed to close all the doors when she heard the military coming down her street. But she wasn’t fast enough. A moment later, she was holding her daughter’s blood soaked body.
“I saw her collapse and initially thought she just slipped and fell. But then blood spurted out from her chest,” she told BBC Burmese from Meiktila in central Myanmar.
It was the randomness of today’s killings that was particularly shocking. Armed with battlefield weapons, the security forces appeared willing to shoot anyone they saw on the streets. The brutality they showed they were capable of today is on another level from what we have seen since the coup.
Neither side – the military or the pro-democracy movement – is willing to back down. The military think they can terrorise people to achieve “stability and security”. But the movement on the streets, led by young people, is determined to rid the country of the military dictatorship once and for all.
It’s painful to have to count the mounting dead, especially the children.
What has the reaction been?
The killings in Myanmar drew international condemnation.
The US embassy said security forces were “murdering unarmed civilians”, while the EU delegation to Myanmar said the 76th Armed Forces Day would “stay engraved as a day of terror and dishonour”.
UN Secretary-General Antonio Guterres said he was “deeply shocked”.
Phil Robertson, deputy director of Human Rights Watch (HRW) in Asia, described the scenes as “shocking, horrible, barbaric and unacceptable”.
What did the coup leader say?
The military has not commented on the killings. In an Armed Forces Day TV address, coup leader Min Aung Hlaing said the army wanted to “join hands with the entire nation to safeguard democracy”.
“Violent acts that affect stability and security in order to make demands are inappropriate,” he said.
He added that the army had to seize power because of “unlawful acts” by democratically-elected leader Ms Suu Kyi and her party.
The military has previously tried to claim that shootings have come from among the protesters.
Armed Forces Day commemorates the start of Myanmar’s military resistance against Japanese occupation in 1945.
A military parade in the capital Naypyitaw was attended by Russia’s Deputy Defence Minister Alexander Fomin. Other countries, including China, Vietnam and Thailand, sent representatives but not ministers, according to Reuters.
Min Aung Hlaing said Russia was a “true friend”.
The US, UK and EU have all imposed sanctions in response to the military coup. Myanmar and Russia’s defence ties have grown in recent years. In that time Moscow has provided training to thousands of soldiers, and has sold arms to the military.