  Microquasars are stellar-mass black holes accreting matter from a companion
star and ejecting plasma jets at almost the speed of light. They are analogues
of quasars that contain supermassive black holes of $10^6$ to $10^{10}$ solar
masses. Accretion in microquasars varies on much shorter timescales than in
quasars and occasionally produces exceptionally bright X-ray flares. How the
flares are produced is unclear, as is the mechanism for launching the
relativistic jets and their composition. An emission line near 511
kiloelectronvolts has long been sought in the emission spectrum of microquasars
as evidence for the expected electron-positron plasma. Transient high-energy
spectral features have been reported in two objects, but their positron
interpretation remains contentious. Here we report observations of $\gamma$-ray
emission from the microquasar V404 Cygni during a recent period of strong
flaring activity. The emission spectrum around 511 kiloelectronvolts shows
clear signatures of variable positron annihilation, which implies a high rate
of positron production. This supports the earlier conjecture that microquasars
may be the main sources of the electron-positron plasma responsible for the
bright diffuse emission of annihilation $\gamma$-rays in the bulge region of
our Galaxy. Additionally, microquasars could be the origin of the observed
megaelectronvolt continuum excess in the inner Galaxy.
