Seven of the nation's largest steel companies, fulfilling months of threats, yesterday filed more than 100 complaints of unfair trade practices against 11 countries.

The filing was followed by a threat from the European Economic Community that the steel dispute might deteriorate into a "serious conflict" and by a vow to fight the complaints.

"There is going to be a very strong reaction," EEC Industrial Affairs Commissioner Etienne Davignon said at a press conference in Brussels. "We are in an extremely tense situation. Protectionism is no longer a risk. It is a probability."

Davignon said the Europeans will fight U.S. steel makers and the EEC will "win the second round." The complaints may disrupt trade relations and increase protectionist sentiments in Europe, the commissioner said.

Davignon also accused U.S. steel makers of filing the complaints to "reduce in a Draconian fashion" European steel imports here, a suggestion not overlooked by some American steel observers.

The complaints could lead to a reduction in steel imports because if the allegations are proven correct, stiff duties could be applied to the imports in the next few months. Importers then would be less likely to buy steel from those firms, steel analysts said.

Steel complaints filed nearly two years ago by U.S. Steel Corp. resulted in a slight dip in steel imports, analysts said.

The complaints filed yesterday cover about half of all carbon steel imported into the United States, but exclude tubular products because of the strong demand for them created by their increased use in oil exploration. With such demand, the presumption is that steel is sold at a fairly high price and isn't dumped, one steel expert said.

The seven steel companies filed "literally truckloads" of complaints and related documents with the Commerce Department and the International Trade Commission early yesterday morning, an ITC spokesman said. U.S. Steel Corp. alone presented the ITC with 400 boxes of materials, the spokesman said.

The steel cases, probably the largest filings the ITC has had, allege dumping, injuring the domestic market by selling steel here at prices below what it cost to make it, and selling government-subsidized steel here--more often called countervailing-duty cases.

The ITC tentatively set Feb. 3 for the first hearing on the complaints. A preliminary ruling from the commission on whether there are reasonable indications the imports are hurting U.S. steel makers is expected before the end of next month.

The complaints were announced after discussions among the EEC, the U.S. government and the U.S. steel makers broke down. U.S. steel leaders said the Europeans didn't provide strong enough assurances that European steelmakers wouldn't flout U.S. trade laws.

In response to the complaints, the Commerce Department said it would suspend the trigger-price mechanism under which the government is supposed to initiate an investigation of steel imports when they are sold here below specified prices.

U.S. Steel filed countervailing-duty and antidumping petitions against Belgium, France, Italy, Luxembourg, the Netherlands, the United Kingdom and West Germany. In addition, countervailing-duty petitions were filed against South Africa and Spain, and antidumping complaints were lodged against Romania.

Bethlehem Steel Corp. filed antidumping and countervailing-duty complaints against West Germany, France, Belgium, Luxembourg, Italy, the Netherlands and the United Kingdom.

Republic Steel Corp., joined by Inland Steel Co., National Steel Corp., Jones and Laughlin Steel Corp. and Cyclops Corp., filed countervailing-duty complaints againts the United Kingdom, France, Belgium, West Germany, Luxembourg, the Netherlands, Italy, Brazil, Spain and South Africa. An antidumping complaint was filed against Romania.