A mobile ad hoc network is a collection of wireless mobile devices communicating with each other and forming a temporary network, without any pre-deployed infrastructure. Security in MANET is a main and important element for the basic functions of a network. One of the security technologies is intrusion detection system which provides a second line of defense. In this paper, we propose an efficient defense system based on a cooperative scheme to deal with intrusions in clustered ad hoc networks. Our proposed system provides security against all network attacks that can be detected by any node in the network, in particular detects the actor. It is simple, reliable, effective and its performance not affected by status of channel.