A key associate of fugitive arms exporter Edwin P. Wilson is being held in a Dallas jail after she was intercepted by U.S. Customs officials upon her arrival on a flight from London.

Assistant U.S. Attorney John Sweeney said yesterday that Roberta Barnes, 33, was being held on $520,000 bond after her arrest Monday night at Dallas-Fort Worth International Airport.

Sweeney said Barnes, who sources here said is in charge of Wilson's London businesses, was detained after she allegedly made a false declaration to customs officials at the airport. She allegedly told the officials she was carrying $5,000 in cash when she had $5,050.

A U.S. magistrate in Washington yesterday issued a material witness warrant for Barnes, who is wanted for questioning in the ongoing investigation into Wilson's activities and those of his former associate, Francis E. Terpil. Barnes has been on a Justice Department "lookout list" of Wilson's associates since her refusal to comply with a grand jury subpoena to testify in the investigation.

Sources familiar with the investigation said Barnes began working for Wilson in Washington in about 1977 and has been running Wilson's London office for at least two years. She reportedly has been handling Wilson's business dealings in London while Wilson has been living in Tripoli, Libya.

Wilson and Terpil, both fugitives, were indicted in April, 1980, by a federal grand jury in Washington on charges of exporting explosives to the radical Libyan regime of Col. Muammar Qaddafi. The grand jury also alleged that Wilson and Terpil ran a school for terrorists in Libya, including instruction on making bombs, and that they attempted to carry out a $1 million assassination contract on a Libyan expatriate.

Another key Wilson associate, Douglas M. Schlachter, was indicted last August in Washington on charges that he coordinated shipments of explosives and terrorist training of Libyan recruits. The grand jury said that Schlacter, who surrendered to federal authorities here last month, allegedly acted under an April, 1976, contract involving Libyan officials and Wilson and Terpil.

Wilson and Terpil are believed to have severed ties, with Terpil last seen living in Beirut.

Barnes, whose passport and other documents were seized by federal officials, is being held pending a Dec. 30 hearing on both the material witness request and the Texas charges. Justice Department officials here are expected to ask that Barnes be sent to testify here.

At a hearing in Dallas yesterday, Barnes said she was on her way to Austin to visit relatives for Christmas.