The prosecution charged yesterday that former mayoral aide Joseph P. Yeldell had corruptly opened doors and unlocked files in the District of Columbia government to benefit millionaire Dominic F. Antonelli Jr. in exchange for Antonelli's secret help in paying off Yeldell's mounting personal debts.

In an opening statement to a newly selected federal jury, the prosecution alleged that Yeldell had brought Antonelli in for meetings with key city officials to help the wealthy developer arrange a highly profitable D.C. government lease. At least twice, the prosecution asserted. Antonelli obtained the city government's own leasing memorandums, apparently through his ties with Yeldell.

In return, the government charged, Antonelli secretly gave Yeldell a $33,000 loan after personally guaranteeing a series of previous "bank loans to Yeldell, amounting to about $21,500. The earlier loans were provided by Madison National Bank in which Antonelli is a major stockholder and a member of the bank's committee that rules on loans.

Raising both arms above his shoulders to underscore his central point, Assistant U.S. Attorney Henry F. Schuelke III told the all-black jury that the government would prove "these parallel actions - Mr. Yeldell on his side, Mr. Antonelli on this [other] side - [were] possible only through their understanding that they would take parallel mutually satisfactory steps." He charged that their aim was to defraud the public for their mutual benefit.

Speaking in a low-key, matter-of-fact tone, Scheulke alleged that Yeldell and Antonelli had carried out their schemes "without regard to the interests of the people of the District of Columbia; rather, they would do so with regard to the interests of Mr. Antonelli and Mr. Yeldell."

The prosecution's allegation, outlined on the second day of Yeldell's and Antonelli's U.S. District Court trial on bribery and conspiracy charges, were immediately countered by Antonelli's chief defense lawyer, Edward Bennett Williams, who portrayed both Antonelli and Yeldell as honest and widely respected men who had never engaged in wrongdoing.

Williams sought to rebut one of the government's key items of evidence, the secret $33,000 loan to Yeldell, by asserting that Antonelli had intended to keep his role in providing the loan a secret even from Yeldell himself.

The prosecution contends that Antonelli had made the loan in the name of a ficticious person - a "straw" - in an attempt to conceal a corrupt deal from the public. Williams argued, however, that Antonelli had relied on the "straw" in an effort to conceal the origin of the loan "so that Mr. Yeldell would have no embarrassment ever in dealing" with the millionaire parking and real estate magnate.

"He wanted Mr. Yeldell not to know that he was the lender," Williams told the jury as he described Antonelli's arrangements for providing the loan. "He took every precaution to make sure that Mr. Yeldell did not know he was the lender." In fact, Williams added, Yeldell did not learn that the money had come from Antonelli until details were later disclosed in newspaper accounts about the Yeldell-Antonelli controversy.

In broad terms, Williams contended that there was no corruption in any of Antonelli's dealings with Yeldell. He described the loans and loan guarantees as an attempt by Antonelli to help a friend "with no expectation of benefit or reward."

He characterized Antonelli's negotiations for the city government lease as having been conducted "fairly and squarely," through "hard" bargaining. In addition, Williams said, the lease Antonelli negotiated was "one of the most - if not the most - advantageous leases" ever obtained by the D.C. government.

"There was no conspriacy between Mr. Antonelli and Mr. Yeldell to defraud the District of Columbia," Williams told the jury. He argued that all their dealings took place "honestly" and "aboveboard."

Yeldell's defense lawyer, John A. Shorter Jr., did not make an opening statement yesterday. He is expected to give one later, after the prosecution has completed presenting its case. The trial, under Judge Gerhard A. Gesell, is expected to continue for three to four weeks.

In his 1 1/2-hour opening statement, Assistant U.S. Attorney Schuelke pointed to a series of events - some previously reported and others disclosed yesterday for the first time - that were intended by the prosecution to depict a corrupt relationship between the financially pressed Yeldell and the profit-minded Antonelli:

A confidential memo laying out the city's lease negotiating strategy, which had been passed on to Yeldell by another official, was alleged to have turned up later in Antonelli's own files.

Yeldell, who at the time headed the city's Department of Human Resources, hired an appraiser chosen by Antonelli to a appraise a building at 60 Florida Ave. NE for which Antonelli was a prime negotiator.

Yeldell temporarily halted negotiations for the Florida Avenue building when he learned that the D.C. Department of General Services was considering leasing it from another businessman, identified an Emanuel Logan.

After obtaining authority for DHR to negotiate its own leases, Yeldell arranged a meeting between Virgil I. McDonald, the agency's newly appointed leasing chief, and Antonelli in Yeldell's own office, and directed McDonald to start lease negotiations with Antonelli for the Florida Avenue building.

After Louis P. Robbins, then principal assistant D.C. corporation counsel, had raised objections to terms of Antonelli's proposed lease for the Florida Avenue building, Yeldell invited Robbins to his office for a meeting with Antonelli. As a result, Robbins was said to have drafted a revised memo that allowed the lease agreement to be concluded.

Antonelli, 56, and Yeldell, 46, stood to face jury yesterday as the panel of nine women and three men was sworn in. The two men, both dressed in light gray, two-piece suits, appeared relaxed throughout the proceedings, as they had also seemed at the start of the trial on Monday.

As Antonelli entered the courtroom yesterday morning, Yeldell's lawyer, Shorter, turned to him to ask, "How are you doing?"

"As well as can be expected," Antonelli replied.

Antonelli spend part of the morning examining financial and other documents expected to be introduced as evidence during the trial. Later he gazed intently at Judge Gesell. Yeldell peered about the courtroom, often watching the jurors faces.

The two men are charged in a three-count grand jury indictment with conspiring in the corrupt award of a $5.6 million. 20-year lease for the two-story Florida Avenue building.

They each face a maximum penalty of five years' imprisonment had a $10,000 fine on the single conspiracy count. They are both also charged with one count of bribery, carrying possible prison terms of up to 15 years and additional fines. The maximum fine on the bribery charge would be either $20,000 or three times the amount of the bribe. It is unclear whether the amount of the bribe would be interpreted to be to be the face value of the $33,000 or some smaller sum.