The G7 called on Myanmar's military to restore power to the democratically elected government after detaining politicians earlier this week.
The foreign ministers of the UK, France, Italy, Japan, Canada, Germany and the US urged the military to end the state of emergency and "release all those unjustly detained and to respect human rights and the rule of law".
The military announced on Monday that it would take power for one year after detaining leader and Nobel Peace Prize winner Aung San Suu Kyi.
The junta accused Ms Suu Kyi's government of not investigating allegations of voter fraud in recent elections, despite international poll watchers saying the race was fair. Her party won last November's vote.
Myanmar has long had a tenuous relationship with democracy. The country was under military rule from 1962 to 2011.
A brief transition period followed and in 2015, the first open elections in 50 years were held.
The military leaders charged Ms Suu Kyi with importing 10 walkie-talkies, an offence that could carry three years in prison.
This is not the first time Ms Suu Kyi has been placed under house arrest. She spent 15 years detained at home under orders from the military leadership and was eventually released in 2010.
Army chief Min Aung Hlaing declared himself the head of a new Cabinet on Monday.
The G7 dismissed the military's accusations of election fraud. "The November election results must be respected and Parliament should be convened at the earliest opportunity," it said.