A UN human rights organization said it had credible information that 18 people were killed and 30 were wounded as Myanmar security forces fired on demonstrators and made mass arrests Sunday to break up protests against the military coup.

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

"Deaths reportedly occurred as a result of live ammunition fired into crowds in Yangon, Dawei, Mandalay, Myeik, Bago, and Pokokku," the United Nations Human Rights Office said in a statement, referring to several cities. It said 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," office spokesperson Ravina Shamdasani said.

The Democratic Voice of Burma reported that as of 5 p.m. in Myanmar, there had been 19 confirmed deaths in nine cities and 10 unconfirmed. The independent media company broadcasts on satellite and digital television, as well as online.

DVB counted five deaths in Yangon and two in Mandalay, the largest and second-largest cities.

It reported five deaths in Dawei, a much smaller city in southeastern Myanmar that has seen tens of thousands of protesters nearly every day since the coup. Witnesses said Sunday’s march was also large, and people were determined not to be driven off the streets.

Confirming the deaths of protesters has been difficult amid the chaos and general lack of news from official sources, especially in areas outside Yangon, Mandalay, and the capital of Naypyitaw. Photos and videos showed killings and bodies.

Gunfire was reported during protests in Yangon, and police also fired tear gas and water cannons while trying to clear the streets. Photos of shell casings from live ammunition used in rifles were posted on social media.

In Dawei, media reported at least three people were killed during a protest march. Photos on social media showed one wounded man in the care of medical personnel.

Before Sunday, there had been eight confirmed reports of killings linked to the army’s takeover, according to the independent Assistance Association of Political Prisoners.

Sunday morning, medical students marched in Yangon near the Hledan Center intersection, which has become a gathering point for protesters who fan out to other parts of the city.

Protesters ran from police and set up makeshift roadblocks to slow the advance of authorities. Some protesters threw tear gas canisters back at the police. Residents pleaded with police to release those they picked up from the street and shoved into trucks to be taken away. 

"The world is watching the actions of the Myanmar military junta and will hold them accountable," said Phil Robertson, deputy Asia director for New York-based Human Rights Watch. "Live ammunition should not be used to control or disperse protests, and lethal force can only be used to protect life or prevent serious injury."

Security forces began employing rougher tactics Saturday, taking preemptive actions to break up protests and making scores, if not hundreds, of arrests. Soldiers joined the police. Many of those detained were taken to Insein Prison on Yangon’s northern outskirts, where political prisoners have been held.

According to the Assistance Association of Political Prisoners, as of Saturday, 854 people had been arrested, charged, or sentenced in relation to the coup, and 771 were detained or sought for arrest.