BRUSSELS, DEC. 3 -- Nearly 30,000 angry farmers coursed through the winding streets of central Brussels today protesting free trade in agriculture as ministers from 107 nations opened week-long negotiations to strengthen world trading rules.

The farmers demonstrated outside the parliament building of the European Community here, throwing stones and tearing down signposts and parking meters before being dispersed by police using tear gas and water cannon.

The anger of the farmers from 24 nations -- largely European, but including a few marchers from the United States and Japan -- provided a vivid example of the political and economic strength of farm blocs and underscored the difficulties negotiators have had over the past four years in coming to grips with the farm trade issue.

The inability of trade officials to agree on ways to control subsidies to farmers under international trade rules, known as the General Agreement on Tariffs and Trade (GATT), now threatens the four-year-old round of talks, which is supposed to end Friday.

Two weeks ago, another band of angry farmers stormed GATT headquarters in Geneva with signs calling the organization's director general, Swiss bureaucrat Arthur Dunkel, "an assassin" and U.S. Trade Representative Carla A. Hills "a witch." Earlier, a South Korean farmer tried to commit suicide on the steps of GATT headquarters to dramatize what he felt was the damage free trade would bring to his country.

"Farmers in most countries have had a rather stable economic existence as a result of government assistance and protection," said U.S. Agriculture Secretary Clayton K. Yeutter. "There is a great resistence to change because of the fear ... even though the greater risk could bring greater reward."

After an opening session of talks today, Yeutter said there had been "no progress" and that there was a "very high level of frustration."

The farm trade dispute has pitted the European Community, which wants to preserve subsidies for its 10 million politically powerful farmers, against the United States and a group of 14 farm exporting nations that favor strict limits on government support payments.

Third World farm exporting nations are threatening to take back concessions they have made in other areas if strong rules are not adopted against farm subsidies.

"We gave up many things on the condition that we can get market access in agriculture," said Ambassador Wylian Otrera of Argentina, whose country's overseas grain sales are undercut by the subsidized products of European Community farmers.

"If we don't get reasonable results in agriculture, we can take those concessions back," he said.

The trade talks, called the Uruguay Round because they were launched in that country in 1986, are aimed at expanding GATT into the trade issues of the 21st century -- high technology, service industries such as banking and engineering, and open investment.

Instead, the talks have been mired in an issue from the past, farm trade, which has been protected all over the world for centuries.

The question is doubly troublesome because it involves not only trade and economic issues, but also cherished myths and the way nations want their societies organized.

Officials of the 12-nation European Community, which is in the process of drawing more closely together to better compete in high technology and manufacturing, maintain that farm subsidies are essential to the social fabric of Western Europe.

Ten percent of all Western Europeans live on farms, compared with 2 percent of the population of the United States. A high EC official said last month that, without subsidies, Europe would lose its bucolic landscape. Economically, Europeans fear that a breakdown of farm subsidies would force workers into cities that already suffer from unemployment.

"We are all small farmers. We would have to leave the land" if subsidies ended, said Michael Murphy, an Irish farmer taking part in today's march. "The fabric of society is at stake," said John Boylan, another Irish farm marcher.

The EC's $12 billion a year in farm subsidies, however, has destabilized farm trade around the world, lowering prices as less efficient European farmers, subsidized by their governments, have been able to make sales that traditionally had gone to other countries.

In Japan, the principle of maintaining its complete ban on rice imports has reached almost mystical proportions, even though rice is seven times as expensive in Japan as on world markets.

Even in the United States, which has led the fight to end farm subsidies, the myth of the family farmer makes it hard to cut subsidies, but the recent budget agreement has made sharp cuts in support payments for farmers.

Major agricultural groups favor the U.S. position at GATT, although specialty interests such as peanut and sugar growers and dairy farmers are likely to work against it.