Images on social media appear to show members of the Black Green Arsonists anarchist group injecting hydrochloric acid into bottles of Coca-Cola, cartons of milk and packets of meat.
Shops across the country have been forced to pull certain products from the shelves amid fears of contamination.
It is thought the group took popular items from shops several weeks ago before injecting them with the acid and discreetly returning the products to the shelves in the runup to Christmas.
Authorities believe the products in question are in circulation in the capital, Athens, and Greece’s second-largest city, Thessaloniki.
Hydrochloric acid is both colourless and odourless as well as being extremely corrosive, and can cause severe damage immediately if ingested.
The contaminated items include 1.5L Coca-Cola and Coca-Cola Light bottles, 350g or 500g packets of popular tube-shaped luncheon meat called Yfantis, and Delta whole milk, according to the US Embassy in Athens.
Almost 2.9 million Britons visited Greece last year and thousands of tourists are thought to be in the country to celebrate the festive period.
While similar threats have been made in the past, this is the first time photographic evidence has emerged.
The group are thought to be eco-anarchists and released the following chilling statement alongside the images: “These days, thousands and thousands of Christians will leave their couch to make the necessary shopping for their Christmas tables, to fill their empty lives with consumable rubbish covered in beautiful, glittering wrapping.
“The victims of this feast are the millions of living creatures that are slaughtered to arrive at the tables of the living, drained to the last drop of blood to satisfy their palates.”
Mary Bossis, an associate professor of International Relations at University of Piraeus and an expert on left-wing militancy said: “I would categorise this as terror activity.
“They are not political activists, they are criminals.”
The Greek Counter Terrorism Unit has opened an investigation into claims of contamination.