The grand swearing-in ceremony was organised inside Wankhede Stadium in South Mumbai on October 31.
The man, identified as Anil Mishra, was spotted on the dais where Modi and other dignitaries were sitting, and police are now investigating how he managed to get there.
"It was brought to our notice that one Anil Mishra, said to be a BJP activist, breached the tight security and reached the dais where PM Narendra Modi ji was sitting along with other dignitaries, without any valid pass," a senior police officer said Thursday evening.
Besides Modi, several top leaders, including BJP president Amit Shah, party patriarch L K Advani, Shiv Sena chief Uddhav Thackeray, several Union ministers, chief ministers of BJP-ruled states and those governed by its alliance partners were present at the high-profile event.
Fadnavis (44) was administered the oath of office and secrecy at the ceremony by Governor C Vidyasagar Rao.
Also sworn-in were seven cabinet ministers and two ministers of state at the sprawling stadium which witnessed a crowd of over 25,000.
The officer said they had been inquiring into the matter and if required an FIR will be registered.
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