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 U.N. human rights official said it had “credible information” that at least 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 February 1 coup. About 1,000 people are believed to have been detained Sunday.

“Deaths reportedly occurred as a result of live ammunition fired into crowds in Yangon, Dawei, Mandalay, Myeik, Bago and Pokokku,” the U.N. 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.

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.

The AP called for his immediate release.

“Independent journalists must be allowed to freely and safely report the news without fear of retribution. AP decries in the strongest terms the arbitrary detention of Thein Zaw,” said Ian Phillips, AP vice president for international news. The Foreign Correspondents Club of Myanmar also condemned the arrest.

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

It registered 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. But in many cases, photos and video circulated showed circumstances of the killings and gruesome photos of bodies.

The independent Assistance Association of Political Prisoners reported it was aware that about 1,000 people were detained Sunday, of whom they were able to identify 270. That brought to 1,132 the total number of people the group has confirmed being arrested, charged or sentenced since the coup.

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

Initial reports on social media identified one young man believed to have been killed. His body was shown in photos and videos lying on a sidewalk until other protesters carried him away.

In Dawei, local media reported at least three people were killed during a protest march, supported by photos and video. 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 Assistance Association of Political Prisoners.

U.N. Secretary-General Antonio Guterres strongly condemned the crackdown, calling the use of lethal force against peaceful protesters and arbitrary arrests “unacceptable,” and expressed serious concern at the increase in deaths and serious injuries, U.N. spokesman Stephane Dujarric said.

“The secretary-general urges the international community to come together and send a clear signal to the military that it must respect the will of the people of Myanmar as expressed through the election and stop the repression,” Dujarric said.

U.S. officials including Secretary of State Antony Blinken also condemned the violence. White House National Security Advisor Jake Sullivan issued a statement saying the U.S. is “alarmed” by the violence and stands in solidarity with Myanmar people “who continue to bravely voice their aspirations for democracy, rule of law, and respect for human rights.”

Washington has imposed sanctions on Myanmar because of the coup, and Sullivan said it would “impose further costs on those responsible,” promising details “in the coming days.”-Photo: BBC Burmese

A wounded man is treated in an ambulance in Yangon's Hledan township.

Protesters run as police fire tear gas during a protest against the military coup in Mandalay.

In this image from a video, medics attend to a man in a street in Dawei, Myanmar, on Sunday.

At least 18 killed and 30 wounded in Myanmar, UN says