About 27 percent of the electorate had voted on Sunday in the first four hours of the fourth round of Bihar's assembly elections, officials said.

Around 27.77 percent of 14 million eligible voters exercised their franchise by 11 a.m. in 55 of the 243 constituencies spread over seven districts, an Election Commission official said.

Twelve of the constituencies are known to be Maoist bastions.

There was no major incident of violence. But police used force when rival political activists clashed outside a polling booth in Sheohar district, officials said.

As many as 776 candidates are in the fray on Sunday in Gopalganj, Siwan, West Champaran, East Champaran, Sheohar, Sitamarhi and Muzaffarpur districts.

The main contest is between Prime Minister Narendra Modi's Bharatiya Janata Party (BJP) and its three allies and the Grand Alliance of Chief Minister Nitish Kumar which has the Janata Dal-United, the RJD and the Congress.

Voters began queueing up even before many polling stations opened. As the day progressed, the voting picked up.

Bihar Police chief P.K. Thakur said barring minor clashes, the polling was on peacefully.

Five helicopters, drones and 1,163 companies of paramilitary forces have been deployed, Additional Chief Electoral Officer R. Lakshaman said.

The polling will end at 3 p.m. in four constituencies and in eight at 4 p.m.

The staggered elections to pick a 243-member Bihar assembly will end on November 5. The result will be known three days later.