A group of trucking companies opposed to deregulation of their industry sued the Interstate Commerce Commission yesterday, charging that the regulatory agency is illegally and improperly constituted.

In a separate development, Transportation Secretary Brock Adams warned that reduced government regulation of the railroad freight industry is essential because of the "pervasiveness" of largely unregulated competition from trucks and barge firms.

The lawsuit was filed in U.S. District Court here by a group called Assure Competitive Transportation, which complained that the ICC now includes only six members when it is constituted to have 11. Trucking firms are being denied due process as a result, the suit contended.

Under Chairman Dan O'Neal, the ICC has been moving to reduce some regulatory controls over the trucking business and freight rates. In addition, the Carter administration deliberately has not appointed some new members to the agency in an attempt to reduce its size -- called unwieldy by critics for many years.

Assure Competitive Transportation earlier moved to have O'Neal disqualified from participating in all deregulation decisions because of his "prejudging the issue" with respect to trucking.

The suit named the ICC, O'Neal and agency members Charles Clapp, Betty Jo Christian, Robert Gresham, Charles Stafford and Virginia Mae Brown.

In a related development, the Federal Trade Commission supported ICC proposals to relax some restrictions on entering the trucking business should be supported.

"Existing restrictions result in higher costs, higher prices and less output than under a free market in trucking," the FTC said in backing the ICC initiative.

The ICC is proposing to change its rules so that any person who wants a trucking license only would have to prove that his firm would serve a useful service. Now, an individual proposing new trucking must guarantee that any new service is not provided by existing firms by presenting new competition.

Secretary Adams told a Senate Transportation subcommittee that, unless ICC controls over the railroad industry are relaxed, taxpayers will end up owning a nationalized system and paying massive subsidies.

Adams said that more than 40 percent of rail freight service today is being supplied by rail companies that "can charitably be described as financially weak." Noting that railroads carry less freight than trucks, Adams said railroads won't survive unless they can compete better.

As a remedy, Adams supported the Carter administration's new rail deregulaton and financial aid package giving railroads greater freedom to set their own prices and to consolidate unnecessary facilities.