Yellow banners marked 'shut down until fish stocks recover' were posted by Greenpeace members near the seven principle entry points to the European Council building - where EU ministers and leaders hold their regular meetings.
At the main entrance, the protesters used cement to build a wall around 20 meters long and two meters high. Fish nets foiled those trying to enter at other points.
EU fisheries and agriculture ministers had been set to meet at the building, in the European quarter of Brussels, until Wednesday.
The meeting is aimed, in part, at setting fishing quotas for each of the EU's 27 member nations for 2008.