The Justice Department filed two civil antitrust suits yesterday challenging the proposed acquisition of Capital Financial Services Inc. of Columbus, Ohio, and Southwest Investment Co. of Amarillo, Tex., by Beneficial Corp. of Wilmington, Del.

The department said that because Beneficial Corp. is the nation's fourth largest independent finance firm, "The acquisitions could substantially lessen competition in making direct cash loans."

The suits charge that the acquisitions would violate Section 7 of the Clayton Act. They were filed in U.S. District Court in Chicago. The complaints ask that the proposed acquisitions be declared unlawful.

But the department didn't request a preliminary injunction to block the acquisitions because Beneficial has agreed that it won't consummate them before Sept. 14 so that the department can study a proposal by Beneficial Corp. to divest certain offices of Capital and Southwest.

The department said this proposal "contemplates the filing of a consent decree." The department added that the proposal "is intended to eliminate the alleged anticompetitive effects of the challenged acquisitions."

According to the Justice Department, Southwestern Investment Co. is a wholly owned subsidiary of Beatrice Foods Co., which also was named in one complaint. The department said Capital Financial Services Inc. is a unit of Continental Corp.'s Buckeye Union Insurance Co. These two concerns were named in the other complaint.

Finn M. W. Casperson, chairman of Benefician Corp., said, "We of course strongly disagree with the government's position."

Casperson also said Beneficial Corp. is engaged in talks with the Justice Department that may result in a disposition of the case acceptable to both sides.