Aung San Suu Kyi, leader of Myanmar's governing National League for Democracy (NLD) party, has been arrested, a party spokesman said.
It comes amid tensions between the civilian government and the military, stoking fears of a coup.
At elections in November, the NLD won enough seats to form a government, but the army says the vote was fraudulent.
Myanmar, also known as Burma, was ruled by the military until 2011. Ms Suu Kyi spent many years under house arrest.
The newly elected lower house of parliament was due to convene for the first time on Monday but the military was calling for a postponement.
The BBC's South East Asia correspondent, Jonathan Head, says there are soldiers on the streets of the capital, Naypyitaw, and the main city, Yangon.
NLD spokesman Myo Nyunt told the Reuters news agency by phone that Ms 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.
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Telephone and internet lines in Naypyitaw have been cut, the BBC's Burmese Service reports.
Soldiers also visited the homes of chief ministers in several regions and took them away, family members said.
On Saturday Myanmar's armed forces promised to abide by the constitution as concerns grew that they were preparing to stage a coup.
