Rep. Charles C. Diggs Jr. (D-Mich.) was described to a federal jury here yesterday as a champion of the "under-represented," a man whose concern for their grievances extended beyond his inner-city Detroit district to persons all over the country.

In his opening statement to the jury in Diggs' trial on charges of illegally diverting congressional funds for his personal use, his lawyers. David Povich, sought to identify Diggs with issues of concern to blacks across the country, in the District of Columbia, and especially in the South.

Povich's opening remarks were interrupted several times by the government's chief prosecutor, John Kotelly, whose objections to the relevance of some of Povich's remarks was upheld by U.S. District Judge Oliver Gasch.

Povich began is statement by conceding that staff aides to Diggs had used portions of their salaries to pay office and personal expenses of the Michigan Democrat.

"That's not what this trial is about," Povich said. Rather, he told the jury of eight women and four men, the question they had to decide was whether it was "a crime and were these expenses paid in violation of the law."

Povich asserted that the payment of Diggs' personal expenses by one of the congressman's aides was voluntary and that the use of salary to pay office expenses, as Diggs did, has since been made legal by the House of Representatives.

Diggs was charged in a 35-count indictment last March with inflating the salaries of three aides, then diverting part of their salaries to pay his personal and office expenses. He also was charged with putting on his congressional payroll persons who performed no official work.

Six counts of the indictment were quietly dropped by the government yesterday as the trial began. Kotelly told reporters that the six counts, all involving one employe on Diggs' payroll, were dropped because it would have been "too convoluted" to prove them.

The jury of 11 blacks and one white person listened attentively to Kotelly's opening statement, which began with a reading of the indictment. Kotelly drily summarized what the government would seek to prove - that Diggs "devised personally the scheme to defraud the United States." Diggs is charged with diverting more than $101,000 in federal funds to his personal use.

Povich told the jury that the "measure of this man has nothing to do with dollars and cents," that Diggs is "constituent-oriented," and that "he did things for people far beyond the 13th District (of Michigan), far beyond the District of Columbia, indeed far beyond the shores of this country."

Povich called the jury's attention to Diggs' prominence as the first black congressman elected from Michigan - he was elected in 1954 - and to his chairmanship of the House District Committee and the International Relations Subcommittee on Africa. But when Povich attempted to describe Diggs' support in 1957 of the Montgomery, Ala, bus boycott and his efforts on behalf of home rule for the District of Columbia, Kotelly objected that these were not relevant and Gasch sustained the objections.

In 1972, when Diggs was "hurting financially." Povich told the jury, he rejected the offer of an "enormous amount of money" from the head of a foreign state that would have freed him from financial concerns. Diggs, Povich said, reported the offer to the State Department. When asked later by reporters, Povich declined to reveal the country or the person involved.

Congressman, Povich told the jury, have considerable discretion in deciding hom much to pay employes. As a result, he said, Diggs legally was able to pay salaries out of his congressional budget to persons who worked for his congressional office and for him personally, or for his funeral business, the House of Diggs, in Detroit.

The trial resumes this morning.