At least two people have been killed and 12 others wounded after police fired at demonstrators protesting against a proposed constitution that would divide Nepal's western region to create new federal states, authorities have said.

Police official Rajesh Lal Karna said on Monday that officers were forced to open fire because hundreds of protesters defied a curfew order and were attacking government buildings in the town of Surkhet.

The constitution, which is in the process of being finalised in the Constituent Assembly and would create six states, has prompted a number of opposition protests across the country this week.

"In western Nepal, they are protesting against the government's decision to lump the midwest with the far western districts and want a federal state of their own," Al Jazeera's Subina Shrestha said, reporting from Nepal.

Meanwhile, many districts of the southern Terai belt have called for an indefinite strike over the provisions for the states, which they say are discriminatory and fail to give enough powers to marginalised communities, Shrestha added.

Police said security would be stepped up on Tuesday and it was still being decided whether the curfew would be extended, as protests were expected to continue.

Nepal has been governed under an interim constitution for seven years.

A new constitution was supposed to be written by a Constituent Assembly elected in 2008, but it failed to finish the task, and another assembly elected in 2012 has also struggled because of disagreements among political parties.