Myanmar's military has taken control of the country after detaining a number of leading politicians and claiming there were "huge discrepancies" in November's election.
A state of emergency has been declared, according to military-owned TV, and power has been handed to the commander-in-chief of the armed forces, Min Aung Hlaing.
Myanmar's leader, Aung San Suu Kyi, was among those detained by the military in an early morning raid on Monday, according to her party.
Phone and internet connections in the capital Naypyitaw and the main commercial centre of Yangon were disrupted, and state TV stopped broadcasting, blaming technical problems.
The Foreign Office warned about possible disruption to ATMs and advised British nationals to "stay home and stay safe".
Myanmar's governing National League for Democracy (NLD) said Ms Suu Kyi was calling on the public not to accept the apparent coup and to protest.
"The actions of the military are actions to put the country back under a dictatorship," the NLD said in a statement which carried Ms Suu Kyi's name and was apparently prepared in advance.
"I urge people not to accept this, to respond and wholeheartedly to protest against the coup by the military."
Earlier, party spokesman Myo Nyunt told the Reuters news agency that Ms Suu Kyi, President Win Myint, and other leaders were "taken" in the early hours of Monday morning.
"I want to tell our people not to respond rashly and I want them to act according to the law," he said, adding that he also expected to be detained.
Tensions between the civilian government and the powerful military have been rising since the election.
Ms Suu Kyi's party won 396 out of 476 seats, but the army claimed the election results were fraudulent - claims that have been rejected by Myanmar's election commission.
But a statement broadcast on military-owned TV on Monday claimed voter lists were "found to have huge discrepancies and the Union Election Commission failed to settle this matter".
It continued: "Although the sovereignty of the nation must derive from the people, there was terrible fraud in the voter list during the democratic general election which runs contrary to ensuring a stable democracy.
"Unless this problem is resolved, it will obstruct the path to democracy and it must therefore be resolved according to the law.
"Therefore, the state of emergency is declared in accordance with article 417 of the 2008 constitution."
Last week, the military declined to rule out a coup to overturn the National League for Democracy's election victory, prompting other countries to warn against "any attempt to alter the outcome of the elections or impede Myanmar's democratic transition".
White House spokeswoman Jen Psaki said: "The United States opposes any attempt to alter the outcome of recent elections or impede Myanmar's democratic transition, and will take action against those responsible if these steps are not reversed."
The Australian government said it was "deeply concerned" and called for the immediate release of the detained leaders.
Japan said it was watching the situation closely but had no plans to repatriate Japanese nationals from Myanmar.
Mark Farmaner, director of Burma Campaign UK, told Sky News that residents of Myanmar were reporting seeing soldiers on the streets but communication was difficult because of the disruption to internet and phone networks.
Mr Farmaner said while Myanmar had been governed by Ms Suu Kyi's civilian administration in recent years, the military had retained control of the most important ministries and security forces.
"It doesn't really make sense for the military to be doing this, because they benefited greatly from the reforms made in the past 10 years," he said of the apparent coup.
"We're going to have to see whether there's some sort of split within the military or what their motivation is.
"They must know as a result of the coup there will be sanctions re-imposed and they face the threat of uprisings within the country. It is impossible to see how this ends well for anybody in the country, including the military."
For decades, Myanmar was ruled by one of the world's most brutal military regimes, and Mr Farmaner said many people inside the country were now "incredibly afraid" that those days would return.
"The people of Burma are the ones who have suffered the most throughout this period and people are now incredibly afraid because they remember the days when there was direct military rule - when there were thousands of political prisoners, when you couldn't speak your mind without being arrested. It looks like they're facing those days again."
John Sifton, Asia advocacy director for Human Rights Watch, said Myanmar's military had never submitted to civilian rule and called on other countries to impose "strict and directed economic sanctions" on the military leadership and its economic interests.
Source: Sky News