Boris Johnson has condemned the coup in Myanmar and demanded that all captured civilian leaders be released.
The UK prime minister condemned the "unlawful imprisonment" of Aung San Suu Kyi and said democracy "must be respected".
He joined a list of other world leaders to have spoken out following the armed forces declaring a state of emergency and detaining a number of leading politicians over claims there were "huge discrepancies" in last November's election.
Ms Suu Kyi's party won 396 out of 476 seats, but the army claimed the election results were fraudulent - allegations that have been rejected by Myanmar's election commission.
Protests broke out near the United Nations University in Tokyo, Japan, with demonstrators carrying signs saying "reject the military" and "we need democracy".
On Monday morning, Mr Johnson tweeted: "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."
Shadow Foreign Secretary Lisa Nandy said there were some "very practical things" that could be done to oppose the military's actions, including "much tighter sanctions".
The Labour MP added: "We really should be going after the military's business interests if we're serious about ramping up the pressure and trying to persuade them to reverse this appalling coup."
In the US, new secretary of state Anthony Blinken expressed "grave concern and alarm" at events in Myanmar, a former British colony that used to be known as Burma and gained independence in 1948.
He said: "We call on Burmese military leaders to release all government officials and civil society leaders and respect the will of the people of Burma as expressed in democratic elections.
"The US stands with the people of Burma in their aspirations for democracy, freedom, peace, and development."
The possibility of the US imposing economic sanctions on Myanmar was floated by US senator Bob Menendez, a Democrat and incoming chair of the foreign relations committee.
Swift criticism also came from EU Commission President Ursula von der Leyen, who said: "I strongly condemn the coup."
She called for the "legitimate civilian government" to be restored and the "immediate and unconditional release of all those detained".
The office of the UN secretary general was also among those to issue a statement condemning the developments as a "serious blow to democratic reforms".
India's foreign ministry said reports Myanmar's commander-in-chief of the armed forces Min Aung Hlaing had taken control prompted "deep concern".
"We believe that the rule of law and the democratic process must be upheld. We are monitoring the situation closely," it said in a statement.
And Amnesty International's deputy regional director for campaigns, Ming Yu Hah, called it an "ominous moment", telling the Myanmar military to clarify "on what legal basis" senior politicians had been rounded up.
She said: "Previous military coups and crackdowns in Myanmar have seen large scale violence and extrajudicial killings by security forces.
"We urge the armed forces to exercise restraint, abide by international human rights and humanitarian law and for law enforcement duties to be fully resumed by the police force at the earliest opportunity."
China's response was more muted.
"We have noted what has happened in Myanmar and are in the process of further understanding the situation," Foreign
Ministry spokesman Wang Wenbin said at a briefing in Beijing.
"China is a friendly neighbour of Myanmar's. We hope that all sides in Myanmar can appropriately handle their differences under the constitution and legal framework and safeguard political and social stability."
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