Suspected militants lobbed a bomb inside the assembly building in Imphal on Saturday, damaging the building. No one was reported injured.
"There were no casualties as the bomb exploded in an area where there were not many people. Some partial damage to the building was caused by the blast," a police official said.
Police are yet to identify the militant group involved in the explosion.
There are more than 19 militant groups active in Manipur, bordering Myanmar, with demands ranging from independence to autonomy.
Thousands of people have lost their lives to insurgency in Manipur during the past two decades.