Video report by ITV News Senior International Correspondent John Irvine
Boris Johnson has said he is "horrified by the escalation of violence" following the killing of dozens of pro-democracy protesters in Myanmar.
On Wednesday, a further 38 people died in demonstrations against the military coup, making it the bloodiest day since the takeover which ousted Aung San Suu Kyi's elected government.
Videos showed security forces shooting a person at point-blank range and chasing down and beating protesters and an ambulance crew.
Despite the violence the day before, protesters returned to the streets on Thursday and were met again with tear gas.
Prime Minister Boris Johnson reacted to the bloodshed in a tweet: "I'm horrified by the escalation of violence in Myanmar and the killing of pro-democracy protesters.
"We stand with the people of Myanmar in calling for an immediate end to military repression, the release of Aung San Suu Kyi and others, and the restoration of democracy."
Videos shared online showing the brutality by security forces has led to international calls for more action.
United Nations (UN) special envoy for Myanmar, Christine Schraner Burgener described Wednesday as “the bloodiest day” since the takeover.
She said: ”I saw today very disturbing video clips. One was police beating a volunteer medical crew. They were not armed.
"Another video clip showed a protester was taken away by police and they shot him from very near, maybe only one meter.
"He didn’t resist to his arrest, and it seems that he died on the street.”
In another video, about two dozen security forces, some with firearms drawn, appear to chase two protesters. When they catch up to them, they repeatedly beat them with rods and kick them. One of the officers is seen is filming the attack on his mobile phone.
The United States said the violence was appalling, the UN human rights chief said it was time to “end the military’s stranglehold over democracy in Myanmar”.
A closed meeting of the UN Security Council - requested by the UK - is expected on Friday.
But any kind of coordinated action by the UN will be difficult since two permanent members of the Security Council, China and Russia, would likely veto it.
Demonstrators have been flooding the streets since the military coup, which reversed years of progress toward democracy after five decades of military rule.