By Charles Haviland
BBC News, Kathmandu
Nepal's parliament has altered the constitution to change the country from a unitary state into a federal one.
Tensions have led to a curfew in Nepalgunj
The amendment was passed overwhelmingly, by 278 votes to five.
The move came as a response to protests from the Madheshi people in southern Nepal who say they are underrepresented and discriminated against.
One man has died in the latest violent demonstrations, and the main town in south-western Nepal has been placed under curfew.
The amendment will also increase the number of constituencies in the troubled southern plains so that for the first time this region, with half the country's population, will have half its parliamentary seats.
A recently-formed group, the Madheshi People's Rights Forum, has been holding strikes and protests for 10 weeks.
There are growing tensions between their activists and those who do not want to strike.
A man died in the south-western town of Nepalgunj after the latest such flare-up. The town has been placed under a 13-hour curfew.
Earlier this year the Forum's demonstrations were often violent and more than 25 people died, most of them killed by the police.
The new constitutional amendment is perhaps over-ambitious.
Apart from federalism, it also promises to give disadvantaged groups like women, ethnic minorities and low castes proportionate representation in all state bodies.
It is not yet clear what kind of federalism this multi-ethnic country will have, but the Madheshi Forum leaders say these changes are not enough.
They want what they call "full autonomy" for southern Nepal and the resignation of the home affairs minister.