Myanmar's National League for Democracy (NLD) Party has called on the public not to accept a coup by the military and urged them 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 the name of leader Aung San Suu Kyi.
"I urge people not to accept this, to respond and wholeheartedly to protest against the coup by the military."
Myanmar's military seized power on Monday in a coup against the democratically elected Suu Kyi, who has been detained along with other leaders of her NLD party.
The army said it had carried out the detentions in response to "election fraud", handing power to military chief Min Aung Hlaing and imposing a state of emergency for one year, according to a statement on a military-owned television station.
The generals made their move hours before parliament had been due to sit for the first time since the NLD's landslide win in a November 8 election viewed as a referendum on Suu Kyi's fledgling democratic government.
Phone lines to the capital Naypyitaw and the main commercial centre of Yangon were not reachable, and state TV went off air.
Soldiers took up positions at city hall in Yangon and internet and phone services in the NLD stronghold were disrupted.
Suu Kyi, Myanmar President Win Myint and other NLD leaders had been "taken" in the early hours of the morning, NLD spokesman Myo Nyunt said by phone.
"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 expected to be arrested himself.
The White House said President Joe Biden had been briefed on the arrest of Suu Kyi.
"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," spokeswoman Jen Psaki said in a statement.
The Australian government said it was "deeply concerned at reports the Myanmar military is once again seeking to seize control of Myanmar" and called for the immediate release of the unlawfully detained leaders.
Suu Kyi, 75, came to power after a 2015 election win following decades of house arrest in a struggle for democracy with Myanmar's junta that turned her into an international icon.
Her global standing was damaged after hundreds of thousands of Muslim minority Rohingya fled army operations into refuge from Myanmar's western Rakhine state in 2017 but she remains hugely popular at home.
Political tensions soared last week when a military spokesman declined to rule out a coup ahead of the new parliament convening on Monday, and military chief Min Aung Hlaing raised the prospect of repealing the constitution.
But the military appeared to backtrack on the weekend, issuing a statement on social media on Sunday saying it would "do everything possible to adhere to the democratic norms of free and fair elections".
Myanmar's election commission has rejected the military's allegations of vote fraud.
In its statement declaring the state of emergency, the military cited the failure of the electoral commission to address complaints over voter lists, its refusal to agree to a request to postpone new parliamentary sessions and protests by groups unhappy over the election.
"Unless this problem is resolved, it will obstruct the path to democracy and it must therefore be resolved according to the law," the statement said, citing an emergency provision in the constitution in the event national sovereignty is threatened.