Police in Myanmar's biggest city have fired tear gas at defiant crowds who returned to the streets to continue protesting Monday against the military's seizure of power a month ago - despite reports security forces killed at least 18 people around the country a day earlier.

The weekslong protests in Myanmar turned violent Sunday. The United Nations Human Rights Office said that at least 18 people were killed over the weekend after military forces clashed with peaceful demonstrators in several cities.

Myanmar military forces said a week ago they would no longer tolerate demonstrations. Security forces reportedly switched to live ammunition to disperse crowds - resulting in several fatalities and more than 30 injuries.

"The people of Myanmar have the right to assemble peacefully and demand the restoration of democracy. These fundamental rights must be respected by the military and police, not met with violent and bloody repression," the United Nations' human rights agency said.

U.N. officials said the use of live ammunition against the nonviolent demonstrators Sunday was a violation of international human rights laws. Police reportedly also used tear gas and stun grenades to disperse the protesters gathering in Mandalay, Dawei and Yangon.

One of the victims of Sunday's violence published a post on Facebook, asking the U.N. to take action.

Reporters on the ground when the shooting started said that one of the military officers told them to "get inside" if they "didn't want to die."

The U.N. said military forces arrested dozens of people Sunday - including at least 85 medical personnel and seven journalists. Since the coup more than 1,000 people have been "arbitrarily arrested," U.N. officials said.

State-run news media said the military was responding to the protests and its actions were to "protect the safety of the people." Military officials said they were tracking down the leaders and were prepared to take "serious action" against them.

"We still need the strongest possible action from the international community to immediately end the military coup," Myanmar's ambassador to the U.N., Kyaw Moe Tun, said in a statement.