NEW YORK (AP) _ United States steel importers - not European steelmakers - are being harmed by government action last week to effectively halt most imports of Common Market steel, according to the leader of the American Institute for Imported Steel Inc.

Fred Lamesch, president of the trade group, on Monday told a news conference the trade action amounted to ''harassment'' of domestic steel importers, who were being forced to foot the financial bill for putting pressure on the Common Market to approve continued restrictions on its steel exports to the United States.

''We feel we are being punished for something we are not a party to,'' Lamesch said. ''We protest this as loudly as we can. The Europeans do not feel this at all, even though it is directed at them.''

But Lamesch said he believed the Common Market as early as mid-week could resolve the problems which prevented it from approving continued quotas on steel exports.

Of the 10 Common Market nations, only Britain has refused to endorse a Nov. 1 proposal which would extend through September 1989 a set of quotas scheduled to expire at the end of this year. The British have indicated they wanted clarification of how the pact would affect exports of unfinished steel products and whether it would allow unilateral U.S. restraint on such products.

Last Thursday, the U.S. Customs Service effectively halted most imports of Common Market steel, except for pipes and tubing, by waiving ''immediate delivery'' privileges for the shipments.

''Immediate delivery'' expedites Customs processing of shipments, allowing steel to be loaded directly from ships to trains or trucks for inland transport. Without it, shipments are delayed for days while Customs examines their documentation, and importers must pay not only to have them offloaded but also reloaded onto land transport, Lamesch said.

Lamesch said the extra costs could not be passed on to producers or customers because the steel was being imported under contracts which already fixed a price.

Lamesch also contended Common Market shipments of steel or any other goods did not leave Europe unless the exporters could show the goods would clear U.S. customs. As such there was little need for Customs to extensively reexamine the documentation once shipments reached U.S. ports.

''It's clearly a matter of putting indirect pressure on,'' Lamesch said.

The proposed Common Market agreement would increase allowable European steel imports to 5.5 percent of the U.S. market total from the present 5.4 percent.

Besides Britain, the Common Market nations are: France, West Germany, Italy, Belgium, the Netherlands, Luxembourg, Denmark, Ireland and Greece.

The agreement is part of the Reagan administration's attempt to cut total steel imports from 25 percent to 18.5 percent of the U.S. market, in order to save domestic steel industry jobs. Steel agreements have been reached with 14 other nations.

The American Iron and Steel Institute last week reported more than 1.3 million tons of steel mill products were imported in October, accounting for 17.3 percent of the domestic market.

Lamesch said he believed the foreign steel industry and Common Market nations which have approved the proposal would pressure the British into endorsing it.

The European Community announced Monday it would call a special ministerial meeting for Wednesday if the British did not approve the agreement by then.

Lamesch said he believed the Common Market would realize that without the quota agreement it could face harsher measures from the United States, such as duties to offset subsidies given by foreign governments to their steel industries.

The AIIS represents importers that handle about 70 percent of the steel imported to the United States.