  During the Zwicky Transient Facility (ZTF) Phase-I operation, 78
hydrogen-poor superluminous supernovae (SLSNe-I) were discovered in less than
three years, making up the largest sample from a single survey. This paper
(Paper I) presents the data, including the optical/ultraviolet light curves and
classification spectra, while Paper II in this series will focus on the
detailed analysis of the light curves and modeling. Our photometry is primarily
taken by the ZTF in the $g,r,i$ bands, and with additional data from other
ground-based facilities and Swift. The events of our sample cover a redshift
range of $z = 0.06 - 0.67$, with a median and $1\sigma$ error (16\% and 84\%
percentiles) $z_{\rm med} = 0.265^{+0.143}_{-0.135}$. The peak luminosity
covers $-22.8\,{\rm mag} \leq M_{g,\rm peak} \leq -19.8$\,mag, with a median
value of $-21.48^{+1.13}_{-0.61}$\,mag. Their light curves evolve slowly with
the mean rest-frame rise time of $t_{\rm rise} = 41.9\pm17.8$\,days. The
luminosity and time scale distributions suggest that low luminosity SLSNe-I
with peak luminosity $\sim -20$\,mag or extremely fast rising events
($<10$\,days) exist but are rare. We confirm previous findings that slowly
rising SLSNe-I also tend to fade slowly. The rest-frame color and temperature
evolution show large scatters, suggesting that the SLSN-I population may have
diverse spectral energy distributions. The peak rest-frame color shows a
moderate correlation with the peak absolute magnitude, i.e. brighter SLSNe-I
tend to have bluer colors. With optical and ultraviolet photometry, we
construct bolometric luminosity and derive a bolometric correction relation
generally applicable for converting $g,r$-band photometry to bolometric
luminosity for SLSNe-I.
