He said the Congress and the BJP were two sides of the same coin as both were not serious about fighting corruption.
He also refuted the charge that he wanted to become president.
"I neither have the capacity nor the desire to become the President of India."
Hazare and Singh have been engaged in a war of words since Team Anna asked people to vote against the Congress in Thursday's Hisar by-poll for not supporting the Lokpal Bill.
He said he had asked people to vote against the Congress, but not in favour of the BJP.
"I want people to vote for an honest and clean person."
He wrote: "Some parties are doing their graduation in corruption, some are at the post-gradate level and others have already got their doctorate degrees."
He also hit back at Singh for claiming that the UPA was instrumental in giving India the Right to Information, saying civil society had campaigned for over a decade and forced the government to bring the law.