The junta-appointed judge "did not respect the law", the National League for Democracy's (NLD) central committee said in a statement.
Win Myint and Aung San Suu Kyi "worked for development in the country during the time they were in power, and they worked according to the law".
The NLD "will continue to work with the people... for the release of all political prisoners, to get rid of the military dictatorship and fight for justice", it said.
The military has cited alleged widespread voter fraud during elections in November 2020, which were won resoundingly by the NLD, as a reason for its coup, which sparked huge protests and a bloody crackdown.
International observers said at the time the polls were largely free and fair.
The NLD was decimated by the February 2021 coup, with many senior members in hiding or in jail.
Myanmar has been in turmoil since, with the junta razing villages and conducting mass extrajudicial killings and air strikes on civilians, according to rights groups.