  We have carried out a systematic analysis of the gamma-ray bursts' (GRBs)
light curves detected in the SPI-ACS experiment onboard the INTEGRAL
observatory aimed to search extended emission. The emission occasionally
recorded after the prompt active phase of a GRB in the form of an emission that
is longer than the active phase and less intense is called the extended one.
Out of the 739 brightest GRBs recorded from 2002 to 2017, extended emission has
been detected in $\sim20\%$ of the individual light curves; its maximum
duration reaches $\sim 10000$ s. Two different types of extended emission have
been revealed. One of them is an additional component of the light curve and is
described by a power law (PL) with an index $\alpha \sim -1$ close to the PL
index of the afterglow in the optical and X-ray bands. The second type can be
described by a steeper PL decay of the light curve typical of the active burst
phase. Extended emission has also been found in the combined light curve of
long GRBs in the individual curves of which no extended emission has been
detected. The PL index of the extended emission in the combined light curve is
$\alpha \sim -2.4$. It is most likely associated with the superposition of
light curves at the active phase; its total duration is $\sim 800$ s.
