Many techniques for verifying invariance properties are limited to systems of moderate size. In this paper, we propose an approach based on assume-guarantee contracts and compositional reasoning for verifying invariance properties of a broad class of discrete-time and continuous-time systems consisting of interconnected components. The notion of assume-guarantee contracts makes it possible to divide responsibilities among the system components: a contract specifies an invariance property that a component must fulfill under some assumptions on the behavior of its environment (i.e. of the other components). We define weak and strong semantics of assume-guarantee contracts for both discrete-time and continuous-time systems. We then establish a certain number of results for compositional reasoning, which allow us to show that a global invariance property of the whole system is satisfied when all components satisfy their own contract. Interestingly, we show that the weak satisfaction of the contract is sufficient to deal with cascade compositions, while strong satisfaction is needed to reason about feedback composition. Specific results for systems described by differential inclusions are then developed. Throughout the paper, the main results are illustrated using simple examples.