The coup derails years of Western-backed efforts to establish democracy in Myanmar, formerly known as Burma, where neighbouring China also has a powerful influence.
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 Nov. 8 election viewed as a referendum on Suu Kyi's fledgling democratic rule.
Phone and internet connections in the capital Naypyitaw and the main commercial centre of Yangon were disrupted and state TV went off air after the NLD leaders were detained.
Suu Kyi, Myanmar President Win Myint and other NLD leaders were "taken" in the early hours of the morning, NLD spokesman Myo Nyunt told Reuters by phone. Reuters was subsequently unable to contact him.
A video posted to Facebook by one MP appeared to show the arrest of another, regional lawmaker Pa Pa Han.
In the video, her husband pleads with men in military garb standing outside the gate. A young child can be seen clinging to his chest and wailing.
Troops took up positions in Yangon where residents rushed to markets to stock up on supplies and others lined up at ATMs to withdraw cash. Banks subsequently suspended services due to poor internet connections.
The detentions came after days of escalating tension between the civilian government and the military in the aftermath of the election.
Suu Kyi's party won 83% of the vote in only the second election since a military junta agreed to share power in 2011.
The pre-written statement uploaded on a NLD Facebook page quoted Suu Kyi as saying such army actions would put Myanmar "back under a dictatorship".
"I urge people not to accept this, to respond and wholeheartedly to protest against the coup by the military," it quoted her as saying. Reuters was unable to reach any NLD officials to confirm the veracity of the statement.
Some pro-military supporters celebrated the coup, parading through Yangon in pickup trucks and waving national flags but pro-democracy activists were horrified.
"Our country was a bird that was just learning to fly. Now the army broke our wings,” said student activist Si Thu Tun.
The White House said President Joe Biden had been briefed on the arrests, while the U.S. embassy in Yangon issued an alert warning U.S. citizens there of the "potential for civil and political unrest".
"The United States stands with the people of Burma in their aspirations for democracy, freedom, peace, and development. The military must reverse these actions immediately," U.S. Secretary of State Antony Blinken said in a statement.
U.N. Secretary-General Antonio Guterres condemned the detention of political leaders and urged the military to "respect the will of the people," a U.N. spokesman said.
The Australian government said it was "deeply concerned at reports the Myanmar military is once again seeking to seize control of Myanmar".
India, Malaysia and Singapore also expressed concern, while major aid donor Japan said it was watching the situation.
LEAD-UP TO COUP
Nobel Peace Prize winner Suu Kyi, 75, came to power after a 2015 election win that followed decades of house arrest and struggle against the junta that made her an international icon.
While still hugely popular at home, her international reputation was damaged after she failed to stop the forced expulsion of hundreds of thousands of Rohingya from western Rakhine state in 2017.
Rohingya refugees in neighbouring Bangladesh also condemned the move.
"We Rohingya community strongly condemn this heinous attempt to kill democracy," Rohingya leader Dil Mohammed told Reuters by phone. "We urge the global community to come forward and restore democracy at any cost."
Military chief Min Aung Hlaing raised the prospect of repealing the constitution in response to election irregularities as tensions soared last week.
The vote faced some criticism in the West for disenfranchising some ethnic groups including Rohingya, but 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.
Daniel Russel, the top U.S. diplomat for East Asia under former President Barack Obama, who fostered close ties with Suu Kyi, described the military takeover as a severe blow to democracy in the region.
"It’s yet another reminder that the extended absence of credible and steady U.S. engagement in the region has emboldened anti-democratic forces," he said.
Human Rights Watch's Asia advocacy director, John Sifton, criticised the initial White House response as “disappointingly weak” and urged a more concerted international reaction.
“The U.S. needs to work with allies to speak more clearly, in unison, in terms of ultimatums, to put the Myanmar military on notice of the specific consequences that will occur if their coup is not reversed," he said.
(Reporting by Reuters staff; Writing by Stephen Coates; Editing by Lincoln Feast) ((email@example.com;))