Garda use of facial recognition technology would be akin to checking everyone's photo ID in a crowd.
That's according to Liam Herrick, executive director of the Irish Council of Civil Liberties (ICCL).
He was speaking as Gardaí may soon have the right to use facial recognition to tackle serious crime.
Justice Minister Helen McEntee looks set to bring legislation before Cabinet.
It is thought the technology would be used, along with artificial intelligence and expanded surveillance, to allow rapid identification of criminals.
Mr Herrick told Pat Kenny at a basic level, the system could be scanning large groups of people looking for a specific individual.
"It is a monitoring, and that has the ability to track you in terms of your face as an individual identifier.
"And then that's matched against a database - so at the tracking level, you're effectively scanning large crowds using artificial intelligence.
"If you think of the human analogy here, it would be like if the Guards were checking everybody in a public space - stopping them and checking their face against their national ID documents - which of course people wouldn't tolerate, and then matching it against a database".
And he says such a system would require a database to compare it to.
"It first of all requires An Garda Síochána in our case, or another law enforcement body, to compile a database of the population's faces.
"[This] is hugely problematic in and of itself - this has led to legal actions across the United States and around the world".
He adds: "Yes, it can potentially catch bad people.
"But it also can have the effect, in the process of it, of conducting mass surveillance on the whole population in a way that we've never seen before".
Mr Herrick believes a pledge to just use such a system for serious criminals will not be enough.
"The Minister talking about it only being used for serious crime: we're not aware of any system which has effectively managed to discriminate between investigations for serious crime and the use of facial recognition technology for mass surveillance.
"There are deep concerns - not just about rights to privacy - but also in terms of how this technology can impact on people's right to be anonymous in a public space, their freedom of movement, their freedom of assembly and their freedom of association".