Ask a landlord-tenant lawyer and get answers ASAP
Hello and welcome! My name is ***** ***** I will try my level best to help with your situation or get you to someone who can.
Did you inspect the property prior to renting it?
If so, were there people living there then?
Does your lease cover the entire building?
Ok, then you would have grounds to sue the landlord for breach of contract for failing to deliver the property vacant as agreed. If you rented the entire building and it was agreed to be vacant in the lease, then the landlord has the duty to deliver an empty building.
So you would have grounds to sue him for breach of contract in small claims court for any damages you have sustained as well as to force him to eject the occupants because if he signed leases with them after signing one with you, then theirs is invalid since he can't rent the same property to two different parties at the same time. Once he signed a lease with you, he no longer has the right to lease to anyone else until your lease expires.
The limit in CA is $10,000 and the clerk will have the summons and complaint forms that you need to file and have the sheriff serve on the landlord to force him to come to court and explain why he is violating your legal rights to use and possession of the entire premises.