  We present optical and UV analysis of the luminous type Ia supernova SN
2012dn covering the period $\sim -$11 to +109 days with respect to the $B$ band
maximum, that occurred on JD 2456132.89 $\pm$ 0.19, with an apparent magnitude
of $m_{B}^\text{max}$ = 14.38 $\pm$ 0.02. The absolute magnitudes at maximum in
$B$ and $V$ bands are $M_{B}^\text{max} = -19.52 \pm 0.15$ and
$M_{V}^\text{max} = -19.42 \pm 0.15$, respectively. SN 2012dn is marginally
luminous compared to normal type Ia supernovae. The peak bolometric luminosity
of $\log L_\text{bol}^\text{max} = 43.27 \pm 0.06$ erg s$^{-1}$ suggests that
$0.82 \pm 0.12$ M$_\odot$ of $^{56}$Ni was synthesized in the explosion. The
decline rate $\Delta m_{15}(B)_\text{true}= 0.92 \pm 0.04$ mag is lower than
that of normal type Ia supernovae, and similar to the luminous SN 1991T.
However, the photometric and spectroscopic behaviour of SN 2012dn is different
from that of SN 1991T. Early phase light curves in $R$ and $I$ bands are very
broad. The $I$ band peak has a plateau-like appearance similar to the
super-Chandra SN 2009dc. Pre-maximum spectra show clear evidence of C\,{\sc ii}
6580 \AA\, line, indicating the presence of unburned materials. The velocity
evolution of C\,{\ sc ii} line is peculiar. Except for the very early phase
($\sim-$13 d), the C\,{\sc ii} line velocity is lower than the velocity
estimated using the Si\,{\sc ii} line. During the pre-maximum and close to
maximum phase, to reproduce observed shape of the spectra, the synthetic
spectrum code {\sc syn++} needs significantly higher blackbody temperature than
those required for normal type Ia events. The photospheric velocity evolution
and other spectral properties are similar to those of the carbon-rich SN
2006gz.
