  Progress in ultrafast electron microscopy relies on the development of
efficient laser-driven electron sources delivering femtosecond electron pulses
to the sample. In particular, recent advances employ photoemission from metal
nanotips as coherent point-like femtosecond low-energy electron sources. We
report the nonlinear emission of ultrashort electron wave packets from a gold
nanotip generated by nonlocal excitation and nanofocusing of surface plasmon
polaritons. We verify the nanoscale localization of plasmon-induced electron
emission by its electrostatic collimation characteristics. With a plasmon
polariton pulse duration below 8 fs at the apex, we identify multiphoton
photoemission as the underlying emission process. The quantum efficiency of the
plasmon-induced emission exceeds that of photoemission from direct apex
illumination. We demonstrate the application for plasmon-triggered
point-projection imaging of an individual semiconductor nanowire at 3 $\mu$m
tip-sample distance. Based on numerical simulations we estimate an electron
pulse duration at the sample below 10 fs for tip-sample distances up to few
micrometers. Plasmon-driven nanolocalized electron emission thus enables
femtosecond point-projection microscopy with unprecedented temporal and spatial
resolution, femtosecond low-energy electron in-line holography, and opens a new
route towards femtosecond scanning tunneling microscopy and spectroscopy.
