Besides party workers, Kumar was joined by dozens of party MPs and legislators at the "dharna" site at the JP roundabout near Gandhi Maidan.
Kumar said the protest was against the BJP-led central government's injustice and betrayal in granting special category status to the state.
"Mr Modi had said in rallies here that we will give special attention, special status to Bihar. Now it's been five months but nothing has happened. He said these things many times during elections and won handsomely too. But now he has forgotten the people of Bihar," Kumar said.
JD-U spokesman Neeraj Kumar said party leaders and workers were staging similar "dharna" at all district headquarters across the state "to expose the double talk of the central government".
He alleged that the central government had cut fund allocations to Bihar in schemes like Sarva Shikhsha Abhiyan, Mahatma Gandhi National Rural Employment Guarantee Act, Indira Awas Yojna and other central sponsored schemes.
Last week, Kumar had said on his Facebook page that the time has come for the people of Bihar to seek "hisab" (account) from the BJP over its betrayal.
Kumar has already made it clear that the special category status would be his main plank in the next year's assembly elections.