Large groups of Janata Dal-United (JD-U) activists, at places running into a few hundred, squatted on rail tracks all over the state, disrupting train traffic.
They also blockaded highways and other key roads across the state.
Nitish Kumar is pressing the Congress-led central government to give "a special status" to help Bihar develop economically.
Nitish Kumar, accompanied by hundreds of slogans-shouting party leaders and supporters, walked from his official residence here to the Gandhi Maidan to stage a day-long sit-in.
Police said JD-U supporters halted over two dozen long-distance trains, stranding passengers at Patna, Gaya, Jehanabad, Bhagalpur, Nalanda, Muzaffarpur, Madhepura, Saharsa and Darbhanga railway stations.
Buses, trucks and other vehicles mostly kept off the roads. In Patna, most markets remained close.
Nitish Kumar has accused the central government of deliberately denying the special category status to Bihar while granting it to Seemandhara, the residual state in Andhra Pradesh.
"It is a betrayal of the people of Bihar by the UPA (United Progressive Alliance) government," Nitish Kumar said.