As networks expand in size and complexity, they pose greater administrative and management challenges. Software-defined networks (SDNs) offer a promising approach to meeting some of these challenges. In this paper, we propose a policy-driven security architecture for securing end-to-end services across multiple SDN domains. We develop a language-based approach to design security policies that are relevant for securing SDN services and communications. We describe the policy language and its use in specifying security policies to control the flow of information in a multi-domain SDN. We demonstrate the specification of fine-grained security policies based on a variety of attributes, such as parameters associated with users and devices/switches, context information, such as location and routing information, and services accessed in SDN as well as security attributes associated with the switches and controllers in different domains. An important feature of our architecture is its ability to specify path- and flow-based security policies that are significant for securing end-to-end services in SDNs. We describe the design and the implementation of our proposed policy-based security architecture and demonstrate its use in scenarios involving both intra- and inter-domain communications with multiple SDN controllers. We analyze the performance characteristics of our architecture as well as discuss how our architecture is able to counteract various security attacks. The dynamic security policy-based approach and the distribution of corresponding security capabilities intelligently as a service layer that enables flow-based security enforcement and protection of multitude of network devices against attacks are important contributions of this paper.