All riders of Shanghai's subway will have four frontal snapshots taken by video monitoring systems installed last month to upgrade subway security, according to a report by eastday.com, Shanghai's Internet news portal, on Tuesday.

The heightened surveillance can help police track and locate law violators quickly, including those suspected of sexual harassment. Incidents of sexual harassment, such as inappropriate photo-taking, are more frequent in the summer months when people tend to dress more lightly, according to local police.

The subway police upgraded security for the city's entire subway system on June 8. Several types of objects carried by passengers are subject to a mandatory check, including big parcels and large containers of liquid.