Attorneys for a Rockville-based jet airplane trading company indicted in a multimillion-dollar tax fraud case here have accused a federal prosecutor and several Internal Revenue Service agents of altering or "creating" documents to prevent dismissal of the case by a federal judge.

The unusual accusations, contained in a massive memorandum filed in court here this week by Omni International Corp., say that an assistant U.S. attorney and at least three IRS agents hid, altered or manufactured key witness interview summaries and then lied about their actions in court.

The U.S. Attorney's Office here vehemently denied the allegations today and said it will be filing a detailed written response next month.

Citing experts in handwriting, type sizes and watermarks on government stationery, Omni attorneys claimed it had evidence showing that the government tinkered with at least 10 documents and "embarked upon a sordid cover-up, a cover-up that entailed alteration of evidence, creation of evidence, suppression of evidence, destruction of evidence, misrepresentations, unethical conduct and repeated perjury."

The allegations, couched in unusually strong terms, come as attorneys on both sides prepare for the trial of Omni, three of its corporate executives and its former accountant and attorney.

Neither U.S. Attorney J. Frederick Motz nor his assistant, Elizabeth Trimble, the prosecutor accused in the filing, would comment on specific allegations but denied any wrongdoing.

Motz, who is expected to be nominated soon for a federal judgeship by President Reagan, said in a formal statement: "We categorically deny that Ms. Trimble or any of the IRS agents committed perjury or obstructed justice. We will answer the specific allegations which have been made in due course in the proper forum."

Motz added, "We believe that it is improper for The Post to publish an article based upon the one-sided representations of the defendants before the government has had an opportunity to fully present its position to the court in the time frame established by the court. The damage which you will cause to the reputations of dedicated government employes by your premature article is incalculable and can never be repaired."

"Obviously there's a whole other side to this story," Trimble said. . . . . "Nothing is as simple as it sounds . . . . I think you're going to get a fairer picture when you read our response."

Trimble, a veteran of the U.S. Attorney's Office here, is considered by colleagues to be a competent and effective prosecutor.

The company, which has handled jet planes for corporate VIPs, Arab sheiks and such celebrities as singer Frank Sinatra and the late rock music idol Elvis Presley, is charged with conspiring to siphon off millions of dollars in income to its subsidiary in Bermuda to avoid paying U.S. taxes.

According to the indictment, between 1974 and 1980, Omni transferred $16.8 million in airplane sales and leasing income to the books of its Bermuda subsidiary, called Euro Airfinance at the time and now called Omni International, Ltd. The firm reported only $91,574 of that income on its U.S. tax returns, the indictment says.

Altogether, the indictment says, Omni reported $4.4 million in taxable income in the fiscal years between 1975 and 1980, though "the taxable income and tax of the corporation . . . were substantially greater than the amounts disclosed."

Omni was indicted in March 1984 after an extensive investigation of the company's far-flung financial operations.

According to court papers, the allegations of prosecutorial misconduct originated when defense attorneys, including two former high-ranking Justice Department tax division prosecutors hired by Omni Vice President Evan T. Barnett, found government witness interview summaries that they said appeared to be altered or written long after witnesses said they were.

The two former federal prosecutors are Cono R. Namorato, who once served as deputy assistant attorney general for the tax division, and Bernard S. Bailor, who was a senior trial attorney in the division. Namorato is also a former IRS agent.

According to papers filed by the attorneys, Trimble and IRS agents had unsuccessfully attempted to persuade an attorney for Omni to cooperate with the government by giving information about Omni tax procedures in exchange for immunity against possible prosecution. In an April 1984 motion to dismiss the charges, Omni claimed that that effort and interviews with witnesses privy to information given the company by its attorneys violated the attorney-client privilege of confidentiality.

This week's Omni filings further claim that Trimble was questioned about the strength of the Omni case by tax division lawyers at Justice Department headquarters in Washington. She directed IRS agents to alter existing interview summaries or create new ones to reflect a heightened desire by the agents to avoid breaching any attorney-client confidentiality, according to papers filed by Omni attorneys.

The attorneys' allegations focus on 10 summaries of interviews conducted by the government, contending that all were altered or written after Omni's motion to dismiss the indictment, not before, as several government witnesses claimed during an evidentiary hear- ing before U.S. District Court Judge Walter Black last summer.

At last summer's evidentiary hearing, Lyndal Shaneyfelt, a retired FBI documents expert called to testify by Omni's attorneys, contradicted claims by IRS agents that the summaries were written in 1983 prior to Omni's motion to dismiss the indictment, according to the Omni papers.

He said watermarks on several of the summary sheets had the date 1984 impressed on them, and those sheets were not delivered to the U.S. Attorney's Office in Baltimore from an Ohio printing plant until late April 1984, according to the Omni papers. Furthermore, they said, Shaneyfelt testified that Trimble's secretary had changed word processing disks during the course of the investigation, and that later disks produced a slightly different comma than the earlier ones, and could therefore prove when certain documents had been typed.

Omni attorneys contended in the papers that several documents said by the government to have been written before Omni moved to dismiss the indictment thus actually had to have been written after that motion was filed.