Hundreds of truck, tractor and taxi drivers are converging on the centre of Brussels on the eve of a crucial European summit.
The farmers and transport workers are protesting at the crippling effect of soaring fuel prices.
Global oil prices have quadrupled in the past seven years, nearing $140 (90 euros) a barrel in Monday's trading.
There is a heavy police presence in Brussels, blocking access to EU sites ahead of an EU leaders' meeting.
Some streets have already been sealed and police have razor-wire barricades ready to throw across roads if necessary, says the BBC's EU correspondent Laurence Peter in Brussels.
He said an army of tractors were lined up in a park next to the national museums, about 1km (half a mile) from the EU Commission building.
Firecrackers exploded and tractors blared their horns.
One farmer said there were about 1,000 tractors at the protest, though there was no independent count available.
"We want a fair price for our milk and meat," said Emmanuel Tassignon, a farmer from Mons in Belgium's Wallonia region.
"The prices of fertiliser and animal feed have gone up, but the retail price for our produce has not."
EU-wide protests
French President Nicolas Sarkozy has said he suspects Ireland's rejection of the Lisbon Treaty is an expression of anger over high fuel and food prices.
He has proposed introducing a cap on petrol taxes and using the additional revenue generated by high oil prices to fund aid for the struggling fisheries and transport sectors.
But the EU says that there is little it can do to combat high fuel costs directly, preferring to target its efforts on longer-term measures to promote alternative fuel sources.
Nevertheless, the summit is expected to address high oil and food prices as well as map a way forward for the Lisbon Treaty.
French fishermen have been protesting for weeks, with Belgian and Italian colleagues also involved.
UK, Spanish, Portuguese and Dutch lorry drivers have also held similar protests.