"The BJP succumbs to threats of RSS and Ramdev and declares Modi as BJP's PM candidate. Do we still call RSS as a Cultural Organisation," Singh said on micro-blogging site Twitter.
Modi was on Friday anointed the PM candidate by the BJP after a go-ahead from the RSS. It ignored the opposition from party patriarch L K Advani to Modi's elevation.
Congress General Secretary Shakeel Ahmad, meanwhile, alleged a larger conspiracy by BJP President Rajnath Singh in Modi's elevation.
He wondered if the move is an attempt to sideline Advani and Leader of the Opposition in Lok Sabha Sushma Swaraj, both of whom were contenders for the PM post till Friday.
"Is Rajnathji, with the help of RSS, using Modiji to sideline Advaniji & Sushmaji for his own chance, when Modiji will be rejected in 2014?" Ahmad said on Twitter.
View: Why Modi still faces a tough road ahead
HUMOUR: What REALLY happened at Modi's coronation
30 things you should know about Narendra Modi
How Modi poses threat as well as opportunity for Cong
Polarisation or development? Narendra Modi's big dilemma