Hundreds of peaceful anti-coup protesters were "barricaded" by Myanmar security forces in apartment buildings in Yangon on Monday, the UN secretary-general said, calling for their release "without violence or arrests".
The Southeast Asian country has been in turmoil since a February 1 coup ousted civilian leader Aung San Suu Kyi and triggered mass protests against the new military junta.
The police and military have responded with an increasingly brutal crackdown on demonstrators, with more than 50 people killed and nearly 1,800 arrested.
As demonstrators across the country sought to paralyse the economy with strike action following a weekend of night raids and arrests, security forces blocked around 200 protesters "from leaving a four-street area" in Sanchaung township in the country's largest city Yangon, according to the UN rights office.
UN Secretary-General Antonio Guterres was following the developments "very closely", particularly in the township "where hundreds of peaceful protesters have been barricaded inside residential apartment complexes for hours", his spokesman Stephane Dujarric told reporters.
Sharp loud bangs could be heard coming from the area, according to an AFP reporter, although it was not immediately clear if the sounds were caused by gunfire or stun grenades.
Repeated screaming was audible in a live Facebook stream.
"I just escaped from Sanchaung," wrote Maung Saungkha, an activist, on Twitter.
"Almost 200 young protesters are still blocked by the police and soldiers there. Local and international community needs to help them now!"
Around 10:00 pm, "police began shooting and making arrests", UN rights office spokeswoman Liz Throssell said, although she said it was "unclear if they were arresting trapped protesters or newly-arrived demonstrators".
The UN and embassies in Yangon -- including those of the United States and former colonial power Britain -- urged security forces to free the demonstrators.
Guterres called for "maximum restraint" and "the safe release of all without violence or arrests", Dujarric told reporters, adding that "many of those trapped are women who were peacefully marching in commemoration of International Women’s Day".
The statement came after three protesters were shot dead on Monday.
In the northern city of Myitkyina, security forces used tear gas and opened fire during street clashes with stone-throwing protesters.
There was grisly footage of bleeding bodies lying on beds as health workers frantically tried to resuscitate them.
One man was also seen lying face down, with part of his skull blown out.
"Two men were shot dead on the spot, meanwhile three others including a woman were shot in the arm," a medic told AFP.
A third protester was shot dead in the town of Pyapon in the Irrawaddy Delta region, an eyewitness and a rescue official told AFP.
- General strike call -
It was also a dark day for independent media in the country, as security forces raided the office of Myanmar Now in Yangon.
The outlet later had its publishing licence revoked, as did independent media Mizzima, DVB, Khit Thit and 7 Day, following an information ministry order, state broadcaster MRTV said.
That followed the closure of banks, stores, shopping malls and some clothing factories Monday after an appeal by trade unions for a general strike to bring the economy to a standstill.
"To continue economic and business activities as usual... will only benefit the military as they repress the energy of the Myanmar people," 18 unions said in a statement.
"The time to take action in defence of our democracy is now."
Unions are seeking to ramp up an ongoing "Civil Disobedience Movement" -- a campaign urging civil servants to boycott working under military rule -- which has already hit state machinery hard.
The impact has been felt at every level of the national infrastructure, with hospital disruptions, empty ministry offices, and banks unable to operate.
The junta has warned that civil servants "will be fired" with immediate effect Monday if they continue to strike.
- International pleas -
The coup and subsequent crackdown have drawn widespread international condemnation as well as sanctions against key military personnel.
Australia's Foreign Minister Marise Payne said her country was suspending its defence cooperation programme with Myanmar, which included English language training.
Canberra is reviewing its aid programme so money is channelled away from government agencies towards not-for-profit organisations.
The military, which denies responsibility for loss of life in the protests, has defended seizing power by alleging widespread electoral fraud in November's elections, which Suu Kyi's party had won in a landslide.