OT for Security Guards - agency is asking the guards to continue duty since there is no replacement guard available

Dear Friends, We have recently moved to 8hrs shift pattern from 12hrs for our security team across all locations, but soon after the change there are many guards who are unwilling to continue duty because of the cut in the salaries and due to this issue sometime the agency is asking the guards to continue duty since there is no replacement guard available, I wanted to know in this case should the OT be paid to the guards or it has to be considered as normal duty, as this is been advised by the agency not demanded by the client. Also i would like to know if there is any law stating about how many hours can a person do maximum number of OT's.

It is standard 8 hrs work per day n OT for duty beyond that
Limitation on OT is laid down in Shops n establishment acts factories act etc depending on applicability

Dear Friend,
As per Security guard board act, After 8 hours of duty OT is applicable and payable double.