Device-to-device (D2D) communications underlaying LTE-Advanced wireless networks reuse cellular frequency spectrum to establish direct links between users without traversing base stations or the cellular network. In this paradigm, there is a need to optimally allocate resources with a view to maximizing the utility, e.g., the total throughput, and mitigating the interference caused by sharing the same spectrum between cellular users (CUs) and D2D pairs. This paper proposes a scheme for optimally allocating transmit power levels and channels to maximize the total number of active D2D pairs and reused channels while minimizing the aggregate transmit power pertaining to CUs and D2D pairs. We consider a multi-cell scenario in which the transmitter and the receiver of each D2D pair can be in the same cell or in two different cells, and each user can simultaneously transmit over multiple reused channels. The optimization is done via a centralized baseband processing in the cloud radio access network (C-RAN) architecture. Simulations show that via our proposed scheme, more users (both cellular users and D2D pairs) can simultaneously communicate and the total system throughput is also significantly increased.