Maintainence charges up society under rera

There is a huge difference between fit out possession and getting a possession after receiving an OC. Fit out possession is basically a possession given to the owner which still has legal or some civil work left. You need to pay the entire amount to the builder while acquiring the fit out possession. On the other hand, OC is a certificate that establishes that the housing apartment in place has the required civic infrastructure, including sanitation, water, electricity and such other basic amenities. Many builders offer fit out possession but, it is generally risky to accept this possession as the legal formalities are still pending. Speaking about the possession time period after fit out possession, there is no definite time period when you might get an OC. At times, you might not even receive an OC if the construction and facilities don’t meet the authority rules and regulation. However, you can always deny to accept the fit out possession until you get the OC and the builder cannot charge you any kind of penalty for this.

Since you have already taken possession, you can now refuse to pay the maintenance for the period prior to physical possession, let the builder prove that he had handed over fit out possession and initiate legal action to recover the amount, you can challenge the same properly or you can approach consumer forum for this unfair trade practice of the builder

Challange payment in the name of "Welcome" and "back-dated" maintenance.

Since you have already taken possession. security guards have "no' authority to stop you / your tenant to entry and occupy. 

Show Builder Buyer Agreement to a local prudent lawyer for better appreciation of documents and necessary proceeding.

You can obtain an order of injunction agaisnt the builder restraining him from preventing the tenant to shift into the flat without paying the maintenance charges by filing a suit for injunction against the builder and can implead the security agency also as a party to the suit.