Aung San Suu Kyi, leader of Myanmar’s governing National League for Democracy (NLD) party, has been arrested, the spokesman for the party said.
It comes amid tensions between the civilian government and the military, stoking fears of a coup.
The NLD won enough seats in parliament to form a government in November, but the army says the vote was fraudulent.
The army has called on the government to postpone convening of parliament, due to take place on Monday.
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.
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.
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.