The Home Ministry has decided that Mr Ambani, 56, is a "national asset" and deserves the 24X7 protection of 28 commandos because of death threats from terror group Indian Mujahideen.
"Security is given only after threat perception assessment," said minister Manish Tewari as the government confronted criticism for its move. Only political leaders like the Prime Minister and Sonia Gandhi have tighter or Z+ security.
"Billionaires get A to Z-class security.Little girls learning A to Z, however, continue to get kicked out of police stations. What priorities," tweeted writer Chetan Bhagat, reflecting the national outrage over the brutal rape of a five-year-old in Delhi, and the police's alleged reluctance to help her family.
Two months ago, a letter threatening to harm Mr Ambani was hand-delivered to his office in Mumbai.
Mr Ambani, who is worth $21.5 billion (around Rs. 1,16,000 crore), may have antagonised the Mujahideen because of his proximity to Gujarat Chief Minister Narendra Modi. In 2002, Mr Modi was in office when communal riots lacerated his state, leaving hundreds of Muslims dead. The politician is the front-runner for the prime ministerial nomination of his party, the BJP.