For future advances in accelerator physics in general and seeding of free electron lasers (FELs) in particular, precise synchronization between low-level RF-systems, photo-injector laser, seed radiation as well as potential probe lasers at the FEL output is required. In this paper, we propose a modular system that is capable of achieving synchronization of various RF- and optical sub-systems with femtosecond precision over distance of several hundreds meters. Typical synchronization methods based on direct photo-detection are limited by detector nonlinearities, which lead to amplitude-to-phase conversion and introduce excess timing jitter. A new synchronization scheme for extraction of low jitter RFsignals from optical pulse trains distributed by modelocked lasers is demonstrated. It is robust against photodetector nonlinearities. The scheme is based on a transfer of timing information into an intensity imbalance between the two output beams from a Sagnac-loop interferometer. As a first experimental demonstration, sub-100 fs timing jitter between the extracted 2-GHz RF-signal and the 100 MHz optical pulse train from a mode-locked Ti:sapphire laser is demonstrated. Numerical simulations show the scaling to sub-femtosecond precision is possible. Together with low-jitter mode-locked lasers and timing stabilized fiber links, this scheme can be applied for large-scale femtosecond timing distribution and synchronization of RF- and optical sub-systems in accelerator and free electron laser facilities.