Sweep follows military questioning of election results with parliament due to sit this week.
Myanmar leader Aung San Suu Kyi and other senior figures from the governing party were detained in a series of early morning raids, the spokesman for the governing National League for Democracy said on Monday.
The move comes after days of escalating tension between the civilian government and the military that had stirred fears of a coup in the aftermath of a November election the army claims was fraudulent.
Spokesman Myo Nyunt told Reuters news agency by phone that Aung San Suu Kyi, President Win Myint and other leaders had been “taken” in the early hours of the morning.
“I want to tell our people not to respond rashly and I want them to act according to the law,” he said, adding he also expected to be detained.
Myanmar’s Parliament, where the military is given a quarter of seats and also has a proxy party, the Union Solidarity and Development Party, was due to sit in the country’s capital Naypyitaw from Monday.
“It does feel like this is the beginning of a military takeover,” Al Jazeera’s Ali Fowle told Al Jazeera from Yangon, noting that politicians from states and regions, as well as prominent political activists had also been detained, while state media appeared to be off-air.
The NLD won the November poll by a landslide, but the military has been waging a months-long campaign to discredit the outcome, despite no firm evidence of wrongdoing. The USDP has demanded a new election supervised by the military, filed nearly 200 complaints, and taking the issue to the Supreme Court.
“It’s at a critical juncture,” said Damien Kingsbury, a Myanmar expert at Deakin University in Australia. “It’s either the end of military involvement in Myanmar politics or it’s a coup. There’s no middle ground on this. This is crunch time.”
Myanmar was a military dictatorship for decades before it began a transition to democracy in 2008.