Megastar Amitabh Bachchan was spotted singing in a Mumbai local train to raise money for cancer patients.

The 73-year-old star accompanied Saurabh Nimbkar, who sings in the locals for raising money and even featured on the TV show "Aaj Ki Raat Zindagi", hosted by the megastar himself.

"Local from VT (Victoria Terminus) to Bhandup to support Saurabh, who sings in locals for charity to cancer... sang with him..." the "Paa" actor tweeted on Sunday night.

T 2059 -Local from VT to Bhandup to support Saurabh, who sings in locals for charity to cancer .. sang with him .. pic.twitter.com/K3WiabhYU7

— Amitabh Bachchan (@SrBachchan) November 15, 2015

Big B wrote on his blog that the money collected by Nimbkar is distributed to the "less fortunate cancer patients and their families in the city" and that this sudden appearance was to let him know that the "support runs even after he departs from the show ("Aaj Ki Raat Zindagi").

"The idea is not to create a splash among the media, or to create credible content for them, or for the subsequent promotion of the show #AKRHZ... the idea is to let him know that he is not alone...that he is appreciated... that our support runs even after he departs from the show," he wrote.

Big B's appearance came as a "surprise" for Nimbkar and for those who "travel daily" on the local train as the whole exercise was "executed without his knowledge".