The space with trees, seats and public art opens on the former super sewer building site near Blackfriars Bridge.
The site, occupied for the construction of the Thames Tideway tunnel, is due to be decommissioned.
A charity says there is a need for the ban to protect the River Thames from further harm.
The firm behind the super sewer says it can reduce sewage spills into the Thames by 95%.
The Thames Tideway Tunnel's first four sites are in use, reducing sewage pollution in the river.
The new "super sewer" aims to stop sewage flowing into the River Thames but adds £25 a year to bills.
Thames Water, Southern Water and Anglian Water have committed to reducing sewage flow.
