Fast radio bursts (FRBs) are highly dispersed radio bursts prevailing in the
universe. The recent detection of FRB~200428 from a Galactic magnetar suggested
that at least some FRBs originate from magnetars, but it is unclear whether the
majority of cosmological FRBs, especially the actively repeating ones, are
produced from the magnetar channel. Here we report the detection of 1863
polarised bursts from the repeating source FRB~20201124A during a dedicated
radio observational campaign of Five-hundred-meter Aperture Spherical radio
Telescope (FAST). The large sample of radio bursts detected in 88 hr over 54
days indicate a significant, irregular, short-time variation of the Faraday
rotation measure (RM) of the source during the first 36 days, followed by a
constant RM during the later 18 days. Significant circular polarisation up to
75\% was observed in a good fraction of bursts. Evidence suggests that some
low-level circular polarisation originates from the conversion from linear
polarisation during the propagation of the radio waves, but an intrinsic
radiation mechanism is required to produce the higher degree of circular
polarisation. All of these features provide evidence for a more complicated,
dynamically evolving, magnetised immediate environment around this FRB source.
Its host galaxy was previously known. Our optical observations reveal that it is
a Milky-Way-sized, metal-rich, barred-spiral galaxy at redshift
z=0.09795+-0.00003, with the FRB residing in a low stellar density, interarm
region at an intermediate galactocentric distance, an environment not directly
expected for a young magnetar formed during an extreme explosion of a massive
star.