Mr Manjhi, who belongs to the community, said all 22 Dalit castes in the state should join hands if they wanted to benefit politically and accused his mentor-turned-adversary Nitish Kumar of dividing them by coining a separate mahadalit category for "political reasons".
Addressing a convention of his newly-launched Hindustani Awam Morcha (Secular), he dubbed the alliance between Mr Kumar and his one-time arch-rival Lalu Prasad of RJD as an "unholy alliance" and said it must be rooted out.
Though Mr Manjhi remained silent on his alliance with BJP in his speech, a political resolution passed by the convention praised the development work of the Narendra Modi government.
"I urge the scheduled castes and scheduled tribes to come together. They are 16 per cent of the population. They must break the caste barrier and forge ahead together for political benefits.
"I came from a deprived community and poor family. Nitish Kumar made me chief minister thinking I would be a rubber stamp... When I began applying my mind, he forced me out. It was an insult," he said referring to his forced removal from the chief minister's chair.
Bihar has a small ST population and Kumar had succeeded in weaning away a big chunk of the SC community in previous elections by showering them with additional sops by clubbing them together as 'mahadalit'.
Mr Manjhi during his tenure brought Paswans under the mahadalit category as well.
Get Breaking news, live coverage, and Latest News from India and around the world on NDTV.com. Catch all the Live TV action on NDTV 24x7 and NDTV India. Like us on Facebook or follow us on Twitter and Instagram for latest news and live news updates.