Activist-politician Arvind Kejriwal today challenged Prime Minister Manmohan Singh and Congress President Sonia Gandhi to a public debate. And it took only a few hours for Congress leader Digvijaya Singh to hit back; he told NDTV that he accepts the invitation. This is two days after Mr Singh in a strong-worded letter asked Mr Kejriwal to come clean on his NGO funds, saying the member of the civil society group India Against Corruption had turned into a 'self-serving ambitious megalomaniac with scant regard for democracy.'