Nobel laureate Aung San Suu Kyi was detained along with other leaders of her National League for Democracy (NLD) party in early morning raids.
"Many people are now in detention. The constitution that the generals wrote, and that they pledged just 48 hours ago to fully abide by, have now been, been overturned. So, it's nothing short of outrageous, deeply disturbing," Tom Andrews told Reuters.
The army said it had carried out the detentions in response to "election fraud," handing power to military chief Min Aung Hlaing and imposing a state of emergency for one year, according to a statement on a military-owned television station.
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 government.