Myanmar's ambassador to London told Reuters he was locked out of the embassy on Wednesday, with sources saying his deputy had shut him out of the building and taken charge on behalf of the military.
The military seized power in Myanmar in a coup in February and have cracked down on pro-democracy protesters.
London ambassador Kyaw Zwar Minn has broken ranks with the ruling junta in recent weeks, calling for the release of detained civilian leader Aung San Suu Kyi.
"I have been locked out," he told Reuters outside the embassy in central London.
"It's a kind of coup, in the middle of London... you can see that they occupy my building," he said, adding that he was talking to Britain's foreign ministry about the situation.
Four diplomatic sources with knowledge of the matter said that deputy ambassador Chit Win had taken over as charge d'affairs and he and the military attaché had locked the ambassador out of the building.
Kyaw Zwar Minn spoke next to the embassy, where police were standing guard. He spoke to protesters on the street outside.
"We are aware of a protest outside the Myanmar embassy in Mayfair, London. Public order officers are in attendance. There have (been) no arrests," police said in a statement.
Last month, ambassador Kyaw Zwar Minn had called for the release of Suu Kyi and ousted President Win Myint, drawing praise for his "courage" from British foreign minister Dominic Raab.
Britain has sanctioned members of Myanmar's military and some of its business interests in wake of the coup, and has demanded the restoration of democracy.
The British foreign office had no immediate comment on the incident.
"This is my building, I need to go inside. That's why I'm waiting here," Kyaw Zwar Minn told Reuters.