Or any form of wide-area surveillance, which isn't necessarily the same as surveillance of mass amounts of people. From an article I read yesterday, the first documented use of FBI domestic surveillance planes was a bribery case that involved a payoff package tossed out of a train in the 30s.
Really any of the modern battlefield surveillance and situational awareness systems could theoretically be turned to the purposes of mass surveillance of civilian populations, given the motivation and resources.
I would guess that system would actually be very bad at mass surveillance. The radar system would be tuned to detect very small movements (the heartbeat), and reject clutter in the category of anything not moving and static (debris).
So in a wide area with lots of movement, it's ability to determine if something was a separate object or person, and interesting on top of that would be severely limited.
A local police department outside one of the worst areas in the Northeast has the same surveillance systems they used in Baghdad. They like to brag about it in the papers.
So, these things do have other uses.