Protesters take cover as they clash with riot police officers during a protest against the military coup in Yangon on Sunday. Reuters

Security forces in Myanmar opened fire and made mass arrests Sunday as they sought to break up protests against the military’s seizure of power, and a UN human rights official said it had "credible information” that 18 people were killed and 30 were wounded.

That would be the highest single-day death toll among protesters who are demanding that the elected government of Aung San Suu Kyi be restored to power after being ousted by a Feb.1 coup.

The Democratic Voice of Burma reported that as of 5pm in Myanmar, there had been 19 confirmed deaths in nine cities, with another 10 deaths unconfirmed.

A pro-democracy protester is detained by riot police officers. Reuters 

The independent media company broadcasts on satellite and digital terrestrial television, as well as online. DVB counted five deaths in Yangon and two in Mandalay, the largest and second-largest cities.

This screengrab shows Lwin Lwin Oo after he was shot while taking part in protest. AFP

Police were out in force early and opened fire in different parts of the biggest city of Yangon after stun grenades, tear gas and shots in the air failed to break up crowds. Soldiers also reinforced police.

A woman holds up a poster against Myanmar military coup. Reuters 

Several wounded people were hauled away by fellow protesters, leaving bloody smears on pavements, media images showed. One man died after being brought to a hospital with a bullet in the chest, said a doctor who asked not to be identified.

"Throughout the day, in several locations throughout the country, police and military forces have confronted peaceful demonstrations, using lethal force and less-than-lethal force that — according to credible information received by the UN Human Rights Office — has left at least 18 people dead and over 30 wounded.

Protesters run after police fired tear gas to disperse them. AFP

"Deaths reportedly occurred as a result of live ammunition fired into crowds in Yangon, Dawei, Mandalay, Myeik, Bago and Pokokku,” the UN Human Rights Office said in a statement referring to several cities, adding that the forces also used tear gas, flash-bang grenades and stun grenades.

"We strongly condemn the escalating violence against protests in Myanmar and call on the military to immediately halt the use of force against peaceful protesters,” its spokesperson Ravina Shamdasani was quoted saying.

An Associated Press journalist was taken into police custody on Saturday morning while providing news coverage of the protests. The journalist, Thein Zaw, remains in police custody.