Continue the construction, the Injunction order was not served on you by the person claiming to be the owner, it was only orally stated, if the same is served directly by the court that means you will have to cease all construction activities if the injunction states this very clearly.
If at all there is a case filed by Mr x that pertains to this property then you along with your seller will be defendant's in this case and through the due process of law you will get a court notice/summons duly served on you.
After this you can contact an advocate to assist you to defend your stand in this case, your seller has to also assist you in this endeavour.
if he uses muscle power or political power, lodge a complaint with the jurisdictional police (police can also be influenced by him) take an acknowledgement, thereafter you file an injunction suit against this person/s and take a restraining order against him from disturbing you or in any manner trying to disposses you from this property in any manner whatsoever.
at any cost do not let go of your physical possession over the plot.