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 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.
Myo Nyunt later told AFP news agency that given the situation, “we have to assume that the military is staging a coup.”
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. State media was also reported to be off-air.
“Due to current communication difficulties we’d like to respectfully inform you that the regular programmes of MRTV and Myanmar Radio cannot be broadcast,” Myanmar Radio and Television said on a post on its Facebook page.
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.
Last week, the situation escalated when military leader Min Aung Hlaing threatened to abolish the constitution. Two days later, the military backtracked saying media had taken Min Aung Hlaing’s comments out of context.
“The Tatmadaw will defend the 2008 Constitution and only act within the boundary of existing laws,” it said on Saturday.
Across Yangon, the country’s biggest city and commercial capital, many people had begun flying the NLD’s red flag in solidarity with the governing party, while banners had also been erected in the streets declaring support for the elected government.
“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, once a British colony, was led by the military for decades before it began a transition to democracy in 2008.