UN envoy to Myanmar Christine Schraner Burgener said there was shocking footage coming out of the country.
Witnesses said security forces opened fire with rubber and live bullets.
Mass protests and acts of civil disobedience have been seen across Myanmar since the military seized power on 1 February.
Protesters have been calling for an end to military rule and the release of the country’s elected government leaders – including Aung San Suu Kyi – who were overthrown and detained in the coup.
The coup and the violent suppression of protests that followed have led to international condemnation, which Myanmar’s military has so far ignored.
Reacting to Wednesday’s deaths, the UK called for a United Nations Security Council meeting on Friday, while the US said it was considering further action against Myanmar’s military.
The latest violence comes a day after Myanmar’s neighbours urged the military to exercise restraint.
‘They just came out and started shooting’
Ms Schraner Burgener said at least 50 people had now been killed “and many wounded” since the coup began.
She said one video clip showed police beating an unarmed volunteer medical crew. Another showed a protester being shot and probably killed on the street, she said.
“I asked some weapons experts and they could verify to me, it’s not clear but it seems that the police weapons like 9mm submachine guns, so live ammunition,” she said.
Reports from inside Myanmar spoke of security forces opening fire on large crowds in a number of cities, including Yangon, with little warning.
Two boys, aged 14 and 17, were among those who were killed, Save the Children said. A 19-year-old woman was also said to be among the dead. [BBC]