The killing of children and unarmed civilians in Myanmar marks a “new low”, Dominic Raab said as a violent crackdown by the military left at least 100 people dead.

Security forces suppressed protests across the country as the ruling junta celebrated the annual Armed Forces Day holiday with a parade in the capital Naypyidaw.

Several demonstrators were shot in the head, according to online news site Myanmar Now, which put the death toll at 114 late on Saturday – making it the deadliest day since the army took power in a coup last month.

Among the victims were a 13-year-old girl who was shot in her house in the city of Meikhtila after armed forces opened fire on rallies in residential areas. A one-year-old boy reportedly suffered an injury to his eye from a rubber bullet in an area of Yangon.

The killings drew international condemnation, with British ambassador Dan Chugg saying the security forces had “disgraced themselves by shooting unarmed civilians.”

“Dozens of innocent people have reportedly been killed, including children,” he added. “At a time of economic crisis, Covid, and a worsening humanitarian situation, today’s military parade and extrajudicial killings speak volumes for the priorities of the military junta.”

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Foreign secretary Dominic Raab tweeted: “Today’s killing of unarmed civilians, including children, marks a new low. We will work with our international partners to end this senseless violence, hold those responsible to account, and secure a path back to democracy.”

“This 76th Myanmar armed forces day will stay engraved as a day of terror and dishonour,” the European Union’s delegation to Myanmar said on Twitter. “The killing of unarmed civilians, including children, are indefensible acts.”

US ambassador Thomas Vajda also issued a statement saying that “security forces are murdering unarmed civilians.” He added: “These are not the actions of a professional military or police force. Myanmar’s people have spoken clearly: they do not want to live under military rule.”

Protesters had vowed to hold more and bigger demonstrations in the days leading up to the holiday, which was formerly known as Resistance Day because it marks the beginning of Burmese resistance to the Japanese occupation in 1945.

In response, the military issued an announcement on state TV on Friday evening warning the public: “You should learn from the tragedy of earlier ugly deaths that you can be in danger of getting shot in the head and back”

Military leader Min Aung Hlaing did not directly refer to the protest movement during his nationally televised Armed Forces Day speech, referring only to “terrorism which can be harmful to state tranquility and social security.”

The general also sought to justify the overthrow of civilian leader Aung San Suu Kyi’s government, accusing it of failing to investigate irregularities in last November’s general election, and repeating that his government would hand over power after holding “a free and fair election”.

Before today’s crackdown, a total of 328 people had been killed by security forces, according to the Association of Political Prisoners.

Also on Saturday evening Myanmar army fighter jets launched airstrikes on a village near the Thai border in territory controlled by the Karen National Union (KNU), the armed ethnic group. It appeared to be retaliation for a KNU attack on an army base earlier in the day.

Additional reporting by agencies