Union Unite said the 38 former Cains employees have not been paid all money owed since news of the decision to close the brewery broke earlier in May.
Bosses said they "deeply regret" the job losses but were forced to close the operation due to a decline in orders from supermarkets.
They said £50m plans to turn the site into a tourist venue are going ahead.
'Draconian and disgraceful'
Redundant workers are protesting outside the brewery-owned Doctor Duncan's pub in Liverpool and asking people to boycott Cains' beer until the situation is resolved.
Unite regional officer Franny Joyce said the workforce had been treated in a "draconian and disgraceful" manner.
She added: "Unite is 100% behind its members. We are calling on the people of Merseyside to support workers in their fight to secure all the money that is owed to them and to boycott all Cains' beers until this happens."
Cains is due to submit plans in July for restoring its Grade II-listed brewery and creating a Brewery Village featuring a hotel, cinema, apartments, parking, artisan food market and spa.
The plans, which would create 800 jobs, have been backed by senior political figures including city mayor Joe Anderson and Liverpool Riverside MP Louise Ellman.
A Cains spokesman said: "We deeply regret the loss of jobs at the site and the manner in which we were forced to make people redundant.
"But the decline in supermarket own label beer orders had a huge impact on our business and the nature of our brewery is such that its operation cannot be scaled down in a simple fashion.
"It is important to emphasise, however, that brewing remains at the heart of our plans for a redevelopment of the site. The existing brewery will be redesigned during the anticipated scheme so that it is more efficient and better suited for future use.
"We are continuing to discuss matters with our advisors about how we best manage winding up the current canning and brewery side of the business."
