At least one police officer has been injured in the clashes with protesters, Ruptly video agency reports from the scene.
The farmers arrived in the EU capital on tractors to express their anger over falling milk and meat prices.
"Prices have gone down 30-40 percent for most farmers and our farms are really going bankrupt like this," Sieta van Keimpema, vice-president of the European Milk Board farmers group, told AP.
According to organizers, at least 6,000 farmers have gathered in Brussels and some 2,000 tractors are currently blocking traffic in and around the city.
"The milk price is under or around 28 cents [per liter, about 0.2 gallons]. And this is not enough even to cover the costs," Heinz Thorwarth, a farmer from Fuchsstadt, southern Germany, told AP.
The farmers also voiced their disagreement with cheap imports from outside the European Union and called for more direct financial help from their governments.
The rally is taking place in front of the EU headquarters, where the union's agricultural ministers have gathered for extraordinary session to discuss the farming crisis.
The European Dairy Association's Alexander Anton said, "The current market situation affects not only the farming community, but also the whole supply chain."
Green MEP Molly Scott Cato added that EU agriculture ministers "clearly need to provide urgent assistance to those farmers hit by the crisis of overproduction and plummeting prices."
Una amiga nos ha pasado este video a corrspnsles Bruselas sobre la manifestaciรณn de agricultores hoy en la capital eu pic.twitter.com/goLy5y2qId
โ Jacobo de Regoyos (@JRegoyos) September 7, 2015
There is not much information offered on the web about Brussels, Belgium other than it has a Boston University campus, it hosts European leadership conferences and it is on track to transform the economy from agriculture to military technology [Link]
Why so secretive?