Jo Smith, communications manager with Argyll and Bute Council, revealed her undercover strategy to public service colleagues during a conference presentation.
The authority has been at loggerheads with residents over a series of issues, including a controversial programme of school closures and problems with local infrastructure.
During the Glasgow event dedicated to social media, Ms Smith said the council had embraced new communication technology, sending live tweets from meetings, carrying out budget consultations online, and securing the Argyll and Bute Council name in case anyone else registered it.
However, she also then spoke of creating "spy accounts", and using them to join groups covertly to establish what critics or opponents of council policy were saying.
One fellow communications professional at the event, who did not wish to be named, said: "Ms Smith gave a powerful presentation about the highs and lows of using social media, but there was a sense of shock in the room when she described how she set up fake accounts on Facebook and Twitter to spy on people and groups who oppose the council.
"From a professional public relations point of view, it is really poor behaviour. You may not like what taxpayers are saying about you online, but you can't pretend to be one of them in order to find out what they are saying.
"Other professional communicators in the room were very uncomfortable about it – not only is it unethical, it was a strange thing to tell other people about."
Lynda Henderson, of the website for argyll.com – which has been a
thorn in the flesh of the council over school closures and other issues
– described the tactic as "underhand", adding: "That is genuinely horrific. It didn't occur to me they would go that far. That's pretty evil."
Another speaker at the event, Gary McGrow, Scottish Health Services Researcher at Scottish Health Council, questioned Ms Smith about the ethics of the practice during a panel session at the time.
He added: "I did ask whether this was ethical, but I don't think it was being used for those purposes. It was being called a spy account, but what it really is is a dummy account.
"A lot of public-sector organisations are struggling with the use of social media. What she was trying to do was find out how other people were using it, as a project, with a name not linked to the council. I don't have any concerns about it now."
Last night the authority said: "Argyll and Bute Council does not use and does not condone the use of covert social media accounts. The council does use social media as a valuable way to engage with communities.
"We regularly use social media to provide residents with information about weather, school closures and traffic. Recently we used web chats as part of our budget consultation.
"Social media allows us to engage with residents on a personal level and is invaluable in an area covering such a large geographic spread."
A spokesman for the Scottish council umbrella organisation Colsa said it did not know of any council using spy accounts.