Indians begin voting on Friday in what will be the world's biggest-ever election, with nearly a billion registered voters. Prime Minister Narendra Modi's Hindu nationalist BJP is seen as the favourite, but faces a challenge from a huge centre-left opposition coalition. We speak to Louise Tillin of the King's College India Institute about what's at stake. She says that at the heart of these elections lies a battle for the future of India's democracy, amid concerns about unequal media coverage, election financing and the arrest of opposition leaders at the start of the campaign.