"He (Narendra Modi) claims himself to be a poor tea seller ... Then why he is trying to throttle the poor by not making public their actual socio-economic condition so that the development strategy could be revised," he said.
Prasad quoting reports in the internet claimed every third person in village was landless and 75 per cent of the population in the country has less than Rs 5000 income per month.
RJD, along with ally JD(U) has been demanding that the census which was conducted across the country be made public.
Prasad's protest fast would be held a day after the Prime Minister Narendra Modi's tentative programme on July 25 to inaugurate IIT Patna and address a rally at Muzaffarpur.
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He said RJD would contemplate an indefinite strike if the caste census was not released after its July 27 shutdown.
Prasad said he would sit on fast either at social leader Loknayak Jayprakash Narayan's residence at Charkha Samiti in the city or under the statue of dalit icon B R Ambedkar.
Replying to a query whether it was true that it had been decided in the past that the caste census would not be released, Prasad disputed it and said "he as member of Lok Sabha was present in Parliament when the decision to conduct the socio-economic caste census was taken.