US President Joe Biden has threatened to reinstate sanctions in Myanmar after the country's military seized power.
Myanmar's army detained Aung San Suu Kyi and other elected leaders, accusing Ms Suu Kyi's party of fraud over its recent landslide election win.
In a statement, Mr Biden said "force should never seek to overrule the will of the people or attempt to erase the outcome of a credible election".
The United Nations and the UK have also condemned the coup.
The US had removed sanctions over the past decade as Myanmar progressed to democracy. Mr Biden said this would be urgently reviewed, adding: "The United States will stand up for democracy wherever it is under attack."
UN Secretary-General António Guterres called the army's move a "serious blow to democratic reforms", as the security council prepared for an emergency meeting. The UN demanded the release of what it said were at least 45 people who had been detained.
In the UK, Prime Minister Boris Johnson condemned the coup and Aung San Suu Kyi's "unlawful imprisonment".
European Union leaders have issued similar condemnations.
China, which has previously opposed international intervention in Myanmar, urged all sides in the country to "resolve differences". Some regional powers, including Cambodia, Thailand and the Philippines, have said it is an "internal matter".
What has happened in Myanmar?
Troops are patrolling the streets and a night-time curfew is in force, with a one-year state of emergency declared. Ms Suu Kyi has urged her supporters to "protest against the coup".
In a letter written in preparation for her impending detention, she said the military's actions would put the country back under a dictatorship.
The military has already announced replacements for a number of ministers.
On the streets of the main city, Yangon, people said they felt their hard-fought battle for democracy had been lost.
One 25-year-old resident, who asked not to be named, told the BBC: "Waking up to learn your world has been completely turned upside down overnight was not a new feeling, but a feeling that I thought that we had moved on from, and one that I never thought we'd be forced to feel again."
Myanmar, also known as Burma, was ruled by the armed forces until 2011, when democratic reforms led by Aung San Suu Kyi ended military rule.
She spent nearly 15 years in detention between 1989 and 2010. She was internationally hailed as a beacon of democracy and received the Nobel Peace Prize in 1991.
But her international reputation suffered severely following an army crackdown on the mostly Muslim Rohingya minority. Former supporters accused her of refusing to condemn the military or acknowledge accounts of atrocities.
How did the coup unfold?
In the early hours of Monday, the army's TV station said power had been handed over to commander-in-chief Min Aung Hlaing.
Ms Suu Kyi, President Win Myint and other leaders of the National League for Democracy (NLD) were arrested in a series of raids. It is not clear where they are being held.
No major violence has been reported. Soldiers blocked roads in the capital, Nay Pyi Taw, and the main city, Yangon. International and domestic TV channels, including the state broadcaster, went off air. Internet and phone services were disrupted. Banks said they had been forced to close.
Later, the military announced that 24 ministers and deputies had been removed, and 11 replacements had been named, including in finance, health, the interior and foreign affairs.
A curfew is now reportedly in effect from 20:00 local time to 06:00 (13:30-23:30 GMT).
The military takeover follows weeks of tensions between the armed forces and the government following parliamentary elections lost by the army-backed opposition.
The opposition had demanded a re-run of the election, raising allegations of widespread fraud that were not backed by the electoral commission.
What has the reaction been in Myanmar?
Michael Ghilezan, a partner of a US law firm who lives in Yangon, told the BBC he had expected military vehicles and protests in the city, but there was instead an eerie calm. "The most common reaction from my Burmese friends has been anger. They feel deeply betrayed by the military and the USDP."
This was reflected in other comments from the streets, although there have been some supporters of the army out waving flags in Yangon.
Theinny Oo, a development consultant, told Reuters: "We had a lawful election. People voted for the one they preferred. We have no protection under the law now."
Many people feared giving their names. One 64-year-old resident of Hlaing township told AFP: "I don't want the coup. I have seen many transitions in this country and I was looking forward to a better future."
Author and historian Thant Myint-U tweeted that a door had opened to a "very different future", and he feared for the millions who had been descending into poverty.