Telecommunication networks and the Internet are growing together. Peer-to-Peer (P2P) services which are originally offered by network providers, like telephony and messaging, are provided through VoIP and Instant Messaging (IM) by Internet service providers, too. The IP Multimedia Subsystem (IMS) is the answer of the telecommunication industry to this trend and aims at providing Internet P2P and multimedia services controlled by the network operators. The IMS provides mobility and session management as well as message routing, security, and billing. This article is a comprehensive version of a conference publication [1] and introduces a Mobile P2P Web Services framework which enables the creation of arbitrary P2P application on mobile devices within the IMS. The terminals are sharing and publishing a Web Service interface, which provides the capability of coupling arbitrary applications using Web Service technologies and the support of the {IMS} infrastructure. The concept, the necessary Web Service adaptations, and extension as well as the interworking of Web Services, {IMS} nodes, and protocols are described and proofed by an exemplary mobile chess game application. In addition, the performance of mobile P2P Web Services is evaluated by means of an analytical analysis.