Boris Johnson has condemned the coup in Myanmar, where armed forces have seized power and detained the country's civilian leader Aung San Suu Kyi.
The PM criticised the "unlawful imprisonment of civilians" and of Ms Suu Kyi and called for their release.
"The vote of the people must be respected," he said.
Myanmar's military is alleging election fraud in November's democratic vote, won in a landslide by Ms Suu Kyi's National League for Democracy.
She urged her supporters "not to accept" the military's intervention and to "protest against the coup".
All authority has been given to the top army commander Min Aung Hlaing and a one-year state of emergency declared, a statement on Myanmar's military TV said.
In a tweet, the UK prime minister joined other world leaders from countries including the US and Australia in condemning the "unlawful imprisonment of civilians".
I condemn the coup and unlawful imprisonment of civilians, including Aung San Suu Kyi, in Myanmar. The vote of the people must be respected and civilian leaders released.— Boris Johnson (@BorisJohnson) February 1, 2021
UK Foreign Secretary Dominic Raab also criticised the coup, saying: "The democratic wishes of the people of Myanmar must be respected, and the National Assembly peacefully re-convened."
A government spokesman said the UK was calling on Myanmar's military to "respect the rule of law and human rights".
Myanmar, also known as Burma, was ruled by the armed forces until 2011, when democratic reforms spearheaded by Ms Suu Kyi ended military rule.
Labour's shadow foreign secretary Lisa Nandy said: "The democratic rights of the people of Myanmar must be upheld.
"All political prisoners should be released and the UK government should consider further sanctions against the Burmese military."