Insect identification: white spotted beetle
It is a longhorn beetle (Cerambycidae), most certainly from the subfamily Lamiinae (flat-faced longhorns). The overall apparence with a downward-pointing face, partially divided eyes, robust build and spined pronotum fits well with Lamiinae.
The beetle in you picture is very similar to species in the genus
Batocera, for instance Batocera rufomaculata (also called Mango trunk/stem borer), and the red spots on the elytra fit relatively well. This species is considered a pest on mango plantations in India and Nepal (Hill, 2008 and Insects in Indian Agrosystems), so it's probably not all that uncommon.

(picture from Insects in Indian Agrosystems, from the National Bureau of Agricultural Insect Resources.)
However, there are probably hundreds of species in Lamiinae found in India, and I cannot give you a certain answer.
The striped elytra with raised lines doesn't fit with the species I've seen in Monochamus (suggested in the other answers), so I don't think that is the correct genus.
References:

Hill. 2008. Pests of Crops in Warmer Climates and Their Control