If someone knows that a certain place is usurped and that using it is unlawful, but he does not know that there is a problem in performing prayers in a usurped place and performs prayers there, then based on obligatory precaution, his prayers are invalid.
Ruling 858.* If someone is compelled to perform an obligatory prayer while he is on or in a mode of transport, or if he wishes to perform a recommended prayer while he is in such a position, then the seat he performs his prayers on, and the wheels of that mode of transportation, fall under the rules of this first condition, i.e. they must not be ruled to be usurped.