Suu Kyi and other leading officials were detained the day before legislators elected in November were due to start a new parliamentary term in office.
"I urge people not to accept this, to respond and wholeheartedly to protest against the coup by the military," according to a statement that carried Suu Kyi's name but not her signature. "Only the people are important."
A handwritten note at the bottom of the statement posted to Facebook by Suu Kyi's National League for Democracy party, or NLD, said it was written before Monday in anticipation of the army's seizure of power.
The NLD won 83 percent of the vote in the November election and the country's election commission has rejected allegations of impropriety. The army has declared a state of emergency for a year and says it will hold elections after that.
The military's leader, Min Aung-Hliang, will now head the government, according to Myawaddy TV, which is controlled by the army. The military said it seized control in response to "election fraud."
It later removed 24 ministers and named 11 replacements to oversee ministries including finance, defense, foreign affairs and interior, according to Reuters. NBC News has not verified the report.
Troops and riot police stood by in Yangon where residents rushed to markets to stock up on supplies while others lined up at ATMs to withdraw cash.
Yangon-based restaurateur Koki Nakajima, 34, told NBC News he saw Burmese army supporters celebrating the coup on flat-bed trucks in the city, while playing loud music.
The military, which wrote Myanmar’s 2008 constitution, gets 25 percent of the country's parliamentary seats, as well as controlling the defense, interior and border ministries.
The announcement on military-owned television cited the country's constitution, which allows the military to take over in times of emergency. The announcer said the coronavirus crisis and the government's failure to postpone November elections were reasons for the emergency.
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The military drafted the constitution in 2008 and retains power under the charter at the expense of democratic, civilian rule. The New York-based international campaigner Human Rights Watch has described the clause as a “coup mechanism in waiting.”
The takeover was swiftly condemned outside of the country. The U.S., the United Kingdom, the European Union, Australia and Singapore all called for Suu Kyi's release.
President Joe Biden said Monday that the military’s actions were a “direct assault” on the country’s transition to democracy and rule of law and said the U.S. would work with its partners to hold to account those responsible for overturning the country’s democratic transition.
“For almost a decade, the people of Burma have been steadily working to establish elections, civilian governance, and the peaceful transfer of power,” he said in a statement, using the country's name until it was changed by the ruling military junta in 1989. “That progress should be respected.”
Biden said America would review its sanction laws and authorities “followed by appropriate action” and called on the international community to press the military to relinquish power, release officials, and to refrain from violence against civilians.
“The United States is taking note of those who stand with the people of Burma in this difficult hour,” he said.
U.N. Secretary-General Antonio Guterres expressed "grave concern regarding the declaration of the transfer of all legislative, executive and judicial powers to the military."
Monday saw pro-Suu Kyi protests take place in Bangkok, where demonstrators have waged a long campaign against military influence in Thai society.
Phil Robertson, Human Rights Watch's deputy Asia director, called the military's justification for the takeover a "manufactured excuse."
"Our worst nightmare came true," he said.
Suu Kyi, 75, a winner of the Nobel Peace Prize, won a landslide election victory in 2015 after 15 years of house arrest.
Her reputation was tarnished after it emerged that hundreds of thousands of Rohingya Muslims fled army persecution in 2017, but she remains popular at home.
Thinzar Shunlei Yi, a human rights activist in Yangon, called what was happing a repeat of history in reference to the NLD's 1990 landslide election.
"The military has used the same tactics when they don't agree with the result of an election," she said.
"We are concerned about how long this coup will take place," she added. "Before when they did it in 1962, it lasted for decades."
Patrick Smith reported from London, Rhea Mogul reported from Hong Kong.
Reuters contributed to this article.
CORRECTION (Feb. 1, 2021, 9:25 a.m.): An earlier version of this article misstated the capital of Myanmar. It is Naypyidaw, not Yangon.