The violence started on Thursday when hundreds of youths protesting the results of Wednesday's election in Kontagora, about 150 km (90 miles) northwest of the Niger state capital Minna, also burned several buildings, including a portion of the emir's palace, witnesses said.
"Police declined to comment on the death toll, but said they had arrested 15 people in connection with the violence," a Reuters correspondent in Minna said.
Trouble started after the state chief electoral officer declared the ruling PDP party's candidate winner of the poll after local election officers earlier said the opposition PRP candidate was the winner.
The bitter rivalry between the People's Democratic Party, which controls the state, and the PRP, the People's Redemption Party, has led to frequent clashes between the groups.
The election was held in other parts of the state on Saturday, but was deferred in Kontagora due to fighting by party supporters.