The world's biggest restaurant chain, which has annual sales of $ 38 billion, is suing Helen Steel and Dave Morris for libel on the grounds that a 1984 pamphlet the two helped publish damaged McDonald's reputation.
The case was originally expected to last a few weeks, but dragged on as the defendants called a long line of witnesses.
The former gardener and postal worker, who are defending themselves because they cannot afford lawyers, have already summoned around 80 witnesses on topics ranging from food packaging to rainforest destruction. Another 60 are scheduled.
It is all part of an exhaustive case to back up allegations they made in the leaflet that McDonald's promotes an unhealthy diet, damages the environment and exploits workers and children.
"We're doing what has to be done. Someone has to stand up to them," Morris, 41, told reporters outside the imposing Royal Courts of Justice in London. The case is being heard by a single judge because it was deemed to be too complicated for a jury.
McDonald's flatly denies the allegations and said it had no choice but to take the case to court because the leaflet was distributed extensively and not just in Britain.
"We felt that if we didn't challenge the allegations, people might think they were true," company spokesman Mike Love said.
McDonald's, which wants the judge to issue a statement dashing the allegations, does not expect to receive significant damages from Morris or Steel, who in any case could not pay.
A small group of demonstrators, one wearing a white sheet covered in empty McDonald's hamburger and drinks cartons, stood shivering outside the courts building to show their support.
The trial, dubbed the McLibel case, has gradually turned into a public relations nightmare for McDonald's. One former employee testified that his branch had been forced to keep on working as sewage leaked out onto the kitchen floor.
The case is estimated to be costing McDonald's more than 5,000 pounds ($ 8,000) a day in legal expenses. It could also end up costing the British taxpayer more than 2.5 million pounds ($ 4 million) by the time it winds up some time next year.
"McDonald's brought this case as part of an attempt to silence its critics. It has totally backfired," said Steel, 30.
"Since the case started five years ago 1.5 million copies of the leaflet have been handed out," added Morris.