Myanmar security troops fired tear gas and surrounded hundreds of anti-junta protesters at two places in Yangon on Wednesday, prompting the US embassy to call for their withdrawal.
In New York, the UN Security Council failed to agree on a statement that would have condemned the coup in Myanmar, instead calling for restraint by the military warning of "further measures".
Talks on the statement would likely continue, diplomats said, after China, Russia, India and Vietnam all suggested amendments late on Tuesday to a British draft.
They included removing references to a coup and the threat of considering further action.
More than 60 protesters have been killed and 1,900 people arrested in the turmoil in Myanmar since the February 1 coup, the Assistance Association for Political Prisoners said.
The military has brushed off condemnation of its actions as it has in the past when outbreaks of protest were forcibly repressed.
On Wednesday, police stormed a Yangon compound that housed railway staff and surrounded hundreds of protesters in North Okkalapa district in another part of the city.
More than 100 people were arrested at the two sites, witnesses said.
Many of the railway staff are part of a civil disobedience movement that has damaged government businesses and included strikes at banks, factories and shops since the army removed Aung San Suu Kyi's elected government.
"We are seeing reports of innocent students and civilians surrounded by security forces in North Okkalapa, as well as arrests," the US embassy said.
"We call on those security forces to withdraw from the area, release those detained and allow people to depart safely."
Police and army officials did not respond to requests for comment.
On Tuesday, Zaw Myat Linn, an official from Ms Suu Kyi's National League for Democracy, died in police custody.
Lynn was second party figure to die in detention in two days.
"He's been participating continuously in the protests," said Ba Myo Thein, a member of the dissolved upper house of parliament.
The cause of death was not clear. In a Facebook Live broadcast before he was detained, Linn urged people to continue fighting the army, "even if it costs our lives".
Police on Tuesday also cracked down on independent media, raiding the offices of two outlets and detaining two journalists.
At least 35 journalists have been arrested since the February 1 coup, Myanmar Now reported, of whom 19 have been released.
Some police have refused orders to fire on unarmed protesters and have fled to neighbouring India, an officer said.
"As the civil disobedience movement is gaining momentum and protests held by anti-coup protesters at different places, we are instructed to shoot at the protesters," four officers said in a statement to police in the Indian city of Mizoram.
"In such a scenario, we don't have the guts to shoot at our own people who are peaceful demonstrators."
The US "repulsed" by the Myanmar army's continued use of lethal force against its people and is continuing to urge the military to exercise "maximum restraint", State Department spokesman Ned Price said on Tuesday.
The army has justified the coup by saying that a November election won by Ms Suu Kyi's party was marred by fraud – a claim rejected by the electoral commission.
The military has promised a new election but has not said when.
The junta has hired an Israeli-Canadian lobbyist for $2 million to "assist in explaining the real situation" of the coup to the US and other countries, documents filed with the US Justice Department show.
Ari Ben-Menashe and his firm, Dickens and Madson Canada, will represent Myanmar's military government in Washington and lobby Saudi Arabia, the UAE, Israel and Russia, and international bodies such as the UN, according to a consultancy agreement.