At least three people were hurt by rubber bullets in the capital Naypyitaw, a doctor said.
The Feb. 1 coup and detention of elected civilian leader Aung San Suu Kyi has brought the largest demonstrations in more than a decade and a growing civil disobedience movement affecting hospitals, schools and government offices.
Witnesses said police fired guns into the air in Naypyitaw as a crowd refused to disperse on the fourth straight day of protests. One witness told Reuters demonstrators ran away as guns were fired into the air.
Police also fired rubber bullets, media reported, and a doctor said three of four wounded people brought to his hospital had been struck by rubber bullets.
Police had earlier fired water cannon at the protesters, who responded by throwing projectiles, the witness said.
Video from the town of Bago, northeast of the commercial hub of Yangon, showed police confronting a large crowd and blasting them with jets from water cannon.
Police arrested at least 27 demonstrators in the second-biggest city of Mandalay, including a journalist, domestic media organizations said.
The unrest has revived memories of almost half a century of direct army rule until the military began a process of withdrawing from civilian politics in 2011, though it never gave up its overall control over the Suu Kyi's civilian government after it won a 2015 election.
That transformation was brought to a halt by the coup that ousted the government as it was preparing to begin its second term in office after Suu Kyi's National League for Democracy (NLD) swept a November election.
"We are so disappointed and so sad whenever we think about why this has befallen us again," Yangon resident Khin Min Soe said of the return of military rule.
Promises on Monday from junta leader General Min Aung Hlaing to eventually hold a new election in his first address since seizing power drew scorn. He repeated unproven accusations of fraud in the election which he used to justify the coup.
"We will continue to fight," youth activist Maung Saungkha said in a statement, calling for the release of political prisoners and the end of "dictatorship."
Activists are also seeking the abolition of a 2008 constitution drawn up under military supervision that gave the generals a veto in parliament and control of several ministries, and for a federal system in ethnically diverse Myanmar.
An older generation of activists who confronted the military in bloody 1988 protests called for strike action by government workers for another three weeks.
The civil disobedience movement, led by hospital workers, has resulted in a plunge in coronavirus tests, official testing figures showed.
Myanmar has suffered one of the worst coronavirus outbreaks in Southeast Asia with 31,177 deaths from more than 141,000 cases.
The US Embassy said it had received reports of an 8 p.m. to 4 a.m. curfew in the two biggest cities, Yangon and Mandalay.
State media signaled possible action against the protests on Monday when it said the public wanted rid of "wrongdoers" and while orders banning gatherings of more than four people have been imposed, there has been no elaboration from authorities.
In his first televised address as junta leader on Monday, Min Aung Hlaing said the junta would form a "true and disciplined democracy," different to previous eras of military rule, which brought years of isolation and poverty.
"We will have a multiparty election and we will hand the power to the one who wins," he said. The electoral commission had dismissed his accusations of fraud in last year's ballot.
Min Aung Hlaing gave no time frame but the junta has said a state of emergency will last one year.
Western governments have widely condemned the coup, although there has been little concrete action to press the generals.
New Zealand has suspended all high-level political and military contact and will ensure aid does not benefit the military and impose a travel ban on its leaders.
A Singapore businessman plans to exit his investment in a Myanmar tobacco firm linked to the military, joining Japanese drinks giant Kirin Holdings which last week scrapped its Myanmar beer alliance.
The U.N. Security Council has called for the release of Suu Kyi and other detainees. The U.N. Human Rights Council will hold a special session on Friday to discuss the crisis at the behest of Britain and the European Union.
Suu Kyi won the Nobel Peace Prize in 1991 for campaigning for democracy and spent nearly 15 years under house arrest.
The 75-year-old faces charges of illegally importing six walkie-talkies and is being held in police detention until Feb. 15. Her lawyer said he has not been allowed to see her. The US State Department said it tried to reach her but was denied.
Suu Kyi remains hugely popular at home despite damage to her international reputation over the plight of the Muslim Rohingya minority.