Defamation

Until its not public, its not defamation but as you know better, if their intentions are to harass you only without any sound reason, you can intimate (not complaint to) police, send a legal notice to them. If By chance of your society is not 'registered' than you can take some more legal actions against them...

Only if that letter is published publicly.

ask them to show CCTV footage

yes you can but there should malafide to your image

There must be publications of the scandalous statement.

हाँ आप मानहानि का दावा कर सकते हैं