S Singh, a senior police official, said on Monday, "Seven people were killed on the spot when the militants fired indiscriminately with automatic weapons."
In a separate attack on Saturday, DHD fighters killed three railway construction workers in the same district, about 300km (200 miles) south of the Assam state capital of Gawahati.
"The militant group had asked railway construction works to be stopped immediately," another police official said.
The fighters said the attacks were a response to an operation by the Indian army which left 12 DHD members dead.
However, the army says it has not conducted any anti-resistance operations in recent days.
The DHD is fighting for an independent homeland for the Dimasa tribe in Assam.
The last three decades of fighting in the region has left thousands dead.
Numerous groups in the region are fighting for separate homelands.