Aung San Suu Kyi's political party is urging Myanmar's people to oppose Monday's apparent coup and any return to "military dictatorship".
The National League for Democracy released a statement on the Facebook page of its party head, Ms Suu Kyi, saying the military's actions were unjustified and went against the constitution and the will of voters.
Myanmar's military has announced it will hold a new election at the end of a one-year state of emergency after it seized control and reportedly detained Ms Suu Kyi.
The announcement on military-controlled Myawaddy TV said that because national stability is in jeopardy, all government functions would be transferred to Senior General Min Aung Hlaing under a provision in the 2008 constitution that was issued under military rule.
The announcement said that once the election is held, the military would hand power to the winner.
A presenter on the station said that the reason for the takeover was in part due to the government's failure to act on the military's claims of voter fraud in last November's election and its failure to postpone the election because of the coronavirus crisis.
The announcement and the declaration of a state of emergency follows days of concern about the threat of a military coup — and military denials that it would stage one — and came on the morning the country's new Parliament session was to begin.
The takeover is a sharp reversal of the partial yet significant progress toward democracy Myanmar has made in recent years following five decades of military rule and international isolation that began in 1962.
It would also be shocking fall from power for Ms Suu Kyi, who led the democracy struggle despite years under house arrest and and won a Nobel Peace Prize for her efforts.
The military's actions received swift and widespread international condemnation.
Prime Minister Boris Johnson said on Twitter: "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."
New US Secretary of State Anthony Blinken issued a statement expressing "grave concern and alarm" over the reported detentions.
"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," he wrote, using Myanmar's former name.
"The United States stands with the people of Burma in their aspirations for democracy, freedom, peace, and development."
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".
The detention of the politicians and cuts in television signals and communication services on Monday were the first signs that plans to seize power were in motion. Phone and internet access to Naypyitaw was lost and Ms Suu Kyi's National League for Democracy party could not be reached. Phone service in other parts of the country was also reported down, though people were still able to use the internet in many areas.
The Irrawaddy, an established online news service, reported that Ms Suu Kyi, who as state counsellor is the nation's top leader, and the country's president, Win Myint, were both detained in the pre-dawn hours. The news service cited Myo Nyunt, a spokesman for the NLD.
Its report said that the party's Central Executive Committee members, lawmakers and regional Cabinet members had also been taken into custody.
A list of other people believed to have been detained, compiled by political activists who asked not to be named for security reasons, included filmmaker Min Htin Ko Ko Gyi, writer Maung Thar Cho, and prominent veterans of the country's 1988 student protest movement, such as Ko Ko Gyi and Min Ko Naing. Their detention could not immediately be confirmed.
Amid the bickering over the allegations, the military last Tuesday ramped up political tension when a spokesman at its weekly news conference, responding to a reporter's question, declined to rule out the possibility of a coup.
Major General Zaw Min Tun elaborated by saying the military would "follow the laws in accordance with the constitution".
Using similar language the Commander-in-Chief, Senior General Min Aung Hlaing, told senior officers in a speech on Wednesday that the constitution could be revoked if the laws were not being properly enforced. Adding to the concern was the unusual deployment of armoured vehicles in the streets of several large cities.
On Saturday, however, the military denied it had threatened a coup, accusing unnamed organisations and media of misrepresenting its position and taking the general's words out of context.
On Sunday, it reiterated its denial, this time blaming unspecified foreign embassies of misinterpreting the military's position and calling on them "not to make unwarranted assumptions about the situation".