A government prosecutor yesterday accused Mary Treadwell, the one-time head of a real estate affiliate of the defunct Youth Pride self-help organization, of forsaking impoverished tenants she was purporting to help while using their rent money to enrich herself.

But the prosecutor's view of the case was sharply disputed by Treadwell's court-appointed attorney, John W. Nields, who told the jury selected yesterday that defense attorneys would instead prove "she didn't steal one penny and she didn't illegally enrich herself."

Assistant U.S. Attorney Stephen R. Spivack, in his opening statement at Treadwell's fraud trial, told a federal court jury of eight women and four men that Treadwell, president of the real estate venture, P.I. Properties Inc., and other officials of the firm used the company's "operating account as though it was their own."

Nields conceded there was theft involved in the management of P.I. Properties, but he accused Robert E. Lee, the firm's general manager who has already pleaded guilty to conspiracy in the case, of stealing money that he said Treadwell later paid back.

The defense attorney also acknowledged that Clifton Terrace, the dilapidated 285-unit apartment complex off 14th Street NW that was owned and managed by P.I. Properties from 1975 to 1978, "was a disaster."

But he laid the blame for that at the doorstep of the federal Department of Housing and Urban Development, which sold the property to the nonprofit P.I. Properties firm. Nields said that HUD did not provide enough money to rehabilitate the deteriorating project.

"Mary Treadwell is a big woman inside," Nields said in describing his client as a caring advocate of the poor. "When she does something good she does it big, and when she makes a mistake she makes a big mistake. She dropped the ball at Clifton Terrace."

Nields, in addition to blaming Lee for Treadwell's misfortunes in connection with the operation of Clifton Terrace, contended that Joan M. Booth, Treadwell's sister and another P.I. Properties official, made the statement to HUD officials that "administrative staff error" led to the use of $1,145 in Clifton Terrace money to pay for Booth's personal legal fees in 1976.

Spivack claimed that Treadwell made the statement, one of the alleged false statements she is accused of making.

Booth has pleaded guilty to conspiracy and tax evasion.

The lawyers' opening statements came after six days of detailed questioning of 175 prospective jurors, a task made necessary because of the extensive publicity the case has generated and because so many Washingtonians dealt with Youth Pride and its spinoff companies before it folded in 1981.

One of the cofounders of Youth Pride was Marion Barry, now mayor of the District of Columbia and Treadwell's husband during part of the time in which the alleged fraud occurred. In addition to fraud, Treadwell is accused of conspiracy, tax evasion and making false statements to federal officials. Barry, who was divorced from Treadwell in 1977, has not been implicated in any wrongdoing in the case.

The attorneys made their final selection of the 12-member jury and six alternates yesterday morning. However, there was no immediate information available about the members of the jury because U.S. District Judge John Garrett Penn, who is presiding, declined to reveal the jurors' names or even their occupations.

Penn has repeatedly warned the jurors that it may take six weeks to hear all the evidence in the case. The evidence is expected to include a great number of financial statements and government reports that federal investigators collected during a 2 1/2-year investigation leading up to a grand jury indictment 15 months ago.

The complexity of the case, and possibly the stuffiness of Penn's courtroom, were quickly exhibited as Spivack made his opening statement. Two or three jurors appeared to doze off, prompting Penn to ask a federal marshal to nudge one juror and hand the woman a cup of water. At another point, the judge loudly tapped his live microphone in an effort to get all the jurors to pay attention.

Spivack traced Treadwell's alleged offenses through the 22-count indictment against her, contending that the purpose of the purported conspiracy by the P.I. Properties officials "was to unjustly and illegally enrich themselves.

"They pledged the security deposit money of the tenants of Clifton Terrace to benefit their other businesses," which Treadwell also headed, Spivack said. "They did everything possible to conceal, to keep HUD auditors, HUD investigators away."

The prosecutor said that "the biggest lie of all" was P.I. Properties' claim that it was a "charitable and educational nonprofit organization."

Spivack said that among other things, Treadwell used $2,200 in Clifton Terrace operating funds to help pay for her wedding ring when she married Ronald S. Williams, a New Jersey accountant, after her divorce from Barry. Williams, now also divorced from Treadwell, pleaded guilty to making false statements to prosecutors in the case and has been fined $5,000.