The decision was "shocking", the Aam Aadmi Party (AAP) leader tweeted, calling it a "murder of constitution" on the eve of Republic Day on January 26.
"BJP lost elections (in the state). Now acquiring power (through) back door," he said.
Arunachal Pradesh had a Congress government but the governor has been accused of engineering a split in the ruling party. The Supreme Court is hearing a host of petitions related to Arunachal Pradesh.