Melt-induced weakening can play critical role for enabling lithospheric deformation in the areas of intense mantle-derived magmatism, such as mid-ocean ridges, rift zones and hot spots. It implies significant reduction in the long-term strength of the deforming lithosphere subjected to frequent rapid melt percolation episodes along planar, sharply localized zones (dykes). Mechanical energy dissipation balance shows that the long-term effective strength of the melt-weakened lithosphere is a strain-averaged rather than a time-averaged quantity. Its magnitude is mainly defined by the ratio between melt pressure and lithostatic pressure along rapidly propagating dykes, which control most of the visco-plastic lithospheric deformation. We implemented governing equations for melt-bearing deforming visco-elasto-plastic lithosphere based on staggered finite difference and marker in cell techniques. We then quantified the lithospheric strength by performing 2D numerical experiments on long-term lithospheric deformation assisted by frequent short-term dyke propagation episodes. The experiments showed that the lithospheric strength can be as low as few MPa and is critically dependent on the availability of mantle-derived melt for enabling frequent episodes of dyke propagation. Viscous-plastic deformation is localized along propagating weak dykes whereas bulk of the lithosphere only deforms elastically and is subjected to large deviatoric stresses. Thus, the low strength of the melt-weakened lithosphere is associated with high volume-averaged deviatoric stress. Possible geodynamic implications include (1) establishing of a global tectono-magmatic plume-lid tectonics regime in the Archean Earth and modern Venus as well as (2) enabling of plume-induced subduction initiation that triggered global modern-style plate tectonics on Earth.