The activist died of canning at Bhamarpura Chowk in Dhanusha district.
The rickshaw-puller was allegedly severely beaten by the police, a statement issued by the Joint Madhesi Movement Committee said.
The local administration had imposed a day-long curfew in parts of the district to control the situation.
The general strike called by the 30-party opposition alliance led by the main opposition United Communist Party of Nepal (Maoist) has affected normal life in different parts of the southern plains.
Nepal's ruling and opposition party alliances are sharply divided over the issue of federalism, as the deadline to draft a new constitution, January 22 is approaching.