Real-time motion estimation has a growing interest for the guidance of interventional procedures in mobile organs. For this purpose, combined magnetic resonance (MR) imaging and ultrasound (US) echography systems can now provide both MR-and US-images, which can be exploited simultaneously for improved target tracking. For this purpose, two tracking strategies can be investigated: While indirect tracking methods rely on a calibration obtained prior to the intervention, direct tracking methods perform the target localization directly on the continuously acquired position. The current paper describes real-time methodological developments designed for the guidance of non-invasive interventional procedures, using a combined MR/US imaging system: A GPU (Graphics Processing Unit) optimized processing pipeline is proposed for both direct and indirect approaches, in conjunction with simultaneous high-frame-rate MR and echography. Experiments on a moving ex-vivo target were performed with MR-guided HIFU (High Intensity Focused Ultrasound) during continuous ultrasound echography. Real-time US echography-based tracking during MR-guided HIFU heating was achieved with heated area dimensions similar to those obtained for a static target.