Another large protest took place in Mandalay, where police shot dead two people on Saturday near a dockyard as security forces were trying to force workers to load a boat.
Security forces have grown increasingly aggressive against the protesters, who have clashed with Myanmar security forces since the military detained de facto leader Aung San Suu Kyi and other high-ranking officials of the civilian government almost three weeks ago.
In Myitkyina town in the north, which has seen confrontations in recent days, people laid flowers for the dead protesters while young people with banners drove around on motorbikes.
Big crowds marched in the central towns of Monywa and Bagan, in Dawei and Myeik in the south and Myawaddy in the east, pictures showed.
"They aimed at the heads of unarmed civilians".
In response to the violent crackdown by the military regime, the United States said that no one should be harmed for exercising their right to dissent, saying that it is "deeply troubled by the fatal shooting of protesters" in Mandalay.
In doing so they end Myanmar's decade-long experiment with democracy after close to 50 years of military rule.
He told a news conference on Tuesday that the army's actions were within the constitution and supported by most people, and he blamed the protesters for instigating violence.
The more than two weeks of protests had been largely peaceful, unlike previous episodes of opposition during almost half a century of direct military rule to 2011.
But if the numbers on Sunday are anything to go by, the violence looks unlikely to silence opposition.
The US is said it is "deeply concerned" by reports that Myanmar security forces have fired on protesters, US State Department spokesman Ned Price said in tweet on Saturday.
Myanmar's anti-coup protests turn deadly again after police open fire
"I am horrified at more loss of life, including a teenage boy in Mandalay, as the ruling junta escalates its brutality in Myanmar", Tom Andrews, the UN's independent investigator for human rights in the Southeast Asian country, said on Twitter. Witness accounts and photos of bullet casings indicated that the security forces used live ammunition, in addition to rubber bullets, water cannons and slingshots.
Police were not available for comment but the state-run Global New Light of Myanmar newspaper said the strikers sabotaged vessels and attacked police with sticks, knives and catapults. Eight policemen and several soldiers were injured, it said. Security forces are stepping up measures to forcibly remove protesters, such as firing water cannons and guns, resulting in deaths and injuries.
Suu Kyi's National League for Democracy (NLD) condemned the violence as a crime against humanity.
The violence in Mandalay comes a day after a young woman became the first confirmed death from the protests. She was shot in the head on February 9 in the capital, Naypyitaw.
Hundreds of people attended her funeral on Sunday. The other was shot in the chest, the report said.
The army seized power after alleging fraud in the November 8 elections that the NLD swept, detaining Suu Kyi and others. The electoral commission had dismissed the allegations of fraud.
A live Saturday night broadcast on Facebook showed the wife of actor Lu Min describing to neighbours how her husband was arrested and taken away from their home shortly after midnight.
Lu Min has been prominent in demonstrations and is one of six celebrities wanted under an anti-incitement law for encouraging civil servants to join in.
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.
The European Union also responded that it strongly condemned violence against peaceful civilian protesters by the military.
UN Secretary General Antonio Guterres said that he condemns the use of deadly violence in Myanmar, saying that the use of lethal force, intimidation and harassment against peaceful demonstrators in unacceptable.
Sanctions have been imposed on the new military leaders by the US, British and Canadian governments, and they and other nations have called for Ms Suu Kyi's administration to be restored.