Over 2,500 farmers from across the EU burned tires and hay outside an emergency meeting of farm ministers.
They sprayed milk from huge canisters, and the cow's udder, on a square close to the meeting. The jittery cow was frightened by firecrackers, sprang loose and chased an office worker down the street.
The farmers used heavy tractors to block major highways into Brussels and streets in the urban center, creating traffic chaos for dozens of miles outside the city for much of the day, keeping tens of thousands of commuters moving at a snail's pace.
Farmers' demonstrations have often been violent in the past and there was a massive police presence that further choked roads throughout the capital.
Officers prevented the farmers from getting too close to the meeting and there was no major violence.
The farmers' major demand _ limiting production through quotas to drive up prices _ met with no success. EU officials said they still intend to gradually create freer markets for European farm products.
"There will be no backtracking," said EU Farm Commissioner Mariann Fischer Boel.
Farmers want regulation to shield them from market fluctuations and have been protesting for weeks, arguing production costs are currently up to twice as high as market prices. They have dumped tens of millions of gallons (liters) of milk into streets and fields to highlight their plight.
"If there is no change by this winter, I will have to stop milking," said Belgian farmer Richard Patrice. "Every day I lose money. It is as if every morning I wake up and I burn a 50 euro ($73) bill."