A security system for shipping containers transported onboard a vessel detects intrusions into the containers and transmits a security message to a land-based security monitoring station that identifies the container and the current geographic location of the vessel. Each container protected by the security system is provided with a cellular telephone that initiates a limited range telephone call transmission in response to an intrusion. Multiple base stations spread over the vessel each receive calls from proximal containers and forwards call messages to the vessel's communication system which retransmits the call information in the form of the security message to the land based monitoring station.