A general strike called by a fringe left party disrupted activity in the capital Kathmandu and other parts of the country Sunday, reported dpa.
The People's Front Nepal called the strike to protest the decision of the Maoists and other major parties to divide the country along a federal structure.
In Kathmandu, many people were caught unaware as People's Front Nepal supporters stopped vehicles and shut down major markets.
Police said the strike supporters set fire to a passenger bus on the western outskirts of the capital, and vandalized about a dozen cars and motorcycles, accusing the drivers of defying the strike.
Police said about two dozen demonstrators were arrested in different parts of the country.
In Rolpa district, about 450 kilometres west of the capital, police clashed with demonstrators trying to forcibly shut down government offices and detained about a dozen people, including district level politicians.
In Siraha south of Kathmandu, police dispersed dozens of demonstrators who tried to block the main highway by burning tyres.
Nepal was declared a federal democratic republic by the country's constituent assembly on 28 May, after voting to abolish the monarchy.
People's Front Nepal opposes the move, saying a federal structure in a small country as Nepal is impractical.