Hundreds of tractors from across Europe rolled into Brussels on Tuesday in a protest spearheaded by hardline farmers' groups against EU green policies, days before elections across the bloc.

Police said around 500 of the vehicles had gathered near the Atomium monument on the city outskirts, rallying to a call by the Dutch-founded group "Farmers Defence Force", with around 1,200 demonstrators present.

It was the latest in a string of farmer protests that have brought Brussels to a halt several times in recent months, but as of mid-afternoon there were no reports of incidents or disruption to traffic.

The FDF movement behind the protest is seen as close to the far-right, which has widely seized on farmer grievances in the run-up to the June 6-9 polls.

The farmers' mainstream pan-European group Copa-Cogeca, present in previous protests, stayed away from the event.

Several hard-right figures -- including from Poland's nationalist Law and Justice (PiS) and the Flemish Vlaams Belang -- were listed among the days' dozen speakers.

France's right-leaning Coordination Rurale farmers union said it was taking part in protest at the environmental demands of the EU's Common Agricultural Policy, as well as over-regulation and free-trade deals it said were "distorting competition".

Past farmer protests in Brussels, part of a months-long movement that roiled Europe early this year, have led to significant concessions in their favour, including a rollback of eco-friendly requirements in order to qualify for EU subsidies.

This time around, the farmers were given permission to demonstrate only in a specific area on the edge of Brussels. Police were deployed to prevent them from approaching the EU institutions at the heart of the capital.