The Chicago meeting, organised by the US food safety watchdog the Food and Drug Administration (FDA) was more than five times over-subscribed, drawing more than 1,000 people to the commercial centre of America's farm and cattle belt, many of them consumers anxious to vent their concerns.
The meeting opened amid fierce criticism of the FDA, which kept members of the public at a hotel more than 15 minutes drive away, a decision which fuelled an angry street protest.
But the latest revelations from the biotechnology industry, on the extent to which GM foods have already penetrated the supermarket shelves, will cause further alarm.
One reason for the high penetration of GM foods in the US is the extensive use of soya derivatives in processed food. More than 60 per cent of soya beans grown in the US is now genetically modified. The figures help to explain why US food producers are so reluctant to accept compulsory labelling: not only do they fear it would deter consumers from buying, it could also prompt a severe disruption in the market if consumer resistance approached European levels.
Yesterday's meeting constituted a sharp turn in the US government's policy of outright support for the biotechnology industry. But it also reflected official worries about what could happen if European-scale protests erupted in the US. The prevailing mood had little of the sunny optimism that has characterised industry pronouncements so far.
The view that Americans are unconcerned about GM foods was disproved by the turnout in Chicago yesterday. Labelling was the most contentious issue, with the industry insisting it was unnecessary and could be misleading, and opponents calling for "transparency" to increase public confidence.
Until recently, worries about GM foods were voiced only by environmental groups. In the last few months, however, one of the main US consumer groups, the Consumers Union, and a growing number of congress members have joined calls for GM food to be labelled, as the European Union is demanding.
A shift in government policy was heralded in the summer by the US agriculture secretary, Dan Glickman, when he warned industry representatives and farmers that they could not force people to buy their produce if they did not want to.
In a move that surprised producers, he also announced a comprehensive review of the approval procedures for GM crops in the US. The turn in the tide of American opinion was reinforced last week when Congressman Dennis Kucinich, a Democrat from Ohio, tabled a bill to require the labelling of genetically modified food.
He said: "If we are what we eat, then consumers must know what they are eating."