BALTIMORE, JAN. 11 -- The federal obstruction of justice trial of former Maryland state senator Clarence M. Mitchell III opened today amid a flurry of allegations of government plots and misconduct as Mitchell renewed assertions that the Reagan administration is seeking to discredit black leaders through selective prosecutions.

Minutes before jury selection started, attorneys for Mitchell asked U.S. District Judge Norman P. Ramsey to delay the trial so that he could look into Mitchell's allegations of an "unofficial" FBI policy to investigate black elected officials throughout the country "without probable cause." The attorneys asked Ramsey to determine if the alleged policy had been applied against Mitchell. Ramsey turned down the request.

A Justice Department spokesman said no such policy exists. "I never heard of any such policy," said John Russell.

Mitchell's lawyers also asked Ramsey to remove himself from the case for possible conflict of interest because of a recently disclosed federal law enforcement tape recording of a telephone conversation in which Mitchell allegedly talked with the judge's wife in 1985. Mitchell also said the tape recording, which did not involve the current Mitchell case, was altered in part to conceal the identity of the judge's wife.

Ramsey ordered all papers sealed in the telephone matter but reportedly denied Mitchell's request that he get out of the case. Mitchell said his lawyers would ask an appellate court to order Ramsey off the case.

Neither Ramsey nor his wife Tucky, a public relations businesswoman, would comment on the tape recording. U.S. Attorney Breckinridge L. Willcox called the allegation of tape-tampering "nonsense."

Meanwhile, a jury of nine whites and three blacks was selected today in the case against Mitchell, 48, a flamboyant former senator from west Baltimore and son of the late civil rights leader Clarence M. Mitchell Jr. He has been faced with a series of five indictments brought by federal prosecutors, ranging from tax evasion and expense padding charges to involvement in the Wedtech scandal in New York.

This week, he faces charges of lying to a grand jury about his business connections with convicted Baltimore drug dealer Melvin (Little Melvin) Williams. The trial is expected to last one to two weeks.

Two months ago, Mitchell and his brother, state Sen. Michael B. Mitchell, were convicted of accepting $50,000 to stop a congressional probe of the Wedtech Corp. Three more trials against Clarence Mitchell are scheduled for this winter.

Today, Mitchell filed an affidavit by a former FBI informant in Atlanta, claiming there was an "unofficial policy" of the FBI in the late 1970s and early 1980s to investigate "prominent elected and appointed black officials in major metropolitan areas throughout the United States."

The affidavit, signed by Hirsch Friedman, 45, now a lawyer and businessman in Atlanta, said the policy was referred to in the Atlanta FBI field office as the "fruhmenschen policy." Fruhmenschen is a German word meaning "early man" or "Stone Age man," according to the affidavit.

The term was used, Friedman said, because the FBI assumed that "black officials were intellectually and socially incapable of governing major governmental organizations and institutions."

After Ramsey threw out Mitchell's request for an inquiry into the alleged policy, Mitchell told reporters on the steps of the federal courthouse that this showed Ramsey's "inability to be impartial . . . . I'm not going to back down. I'm going to fight this tooth and nail."

In a telephone interview from Atlanta, Friedman acknowledged signing the "fruhmenschen" affidavit but said only one FBI agent in the Atlanta field office, John C. McAvoy, ever used the term. Friedman added that he did not know if the alleged policy originated at FBI headquarters in Washington or was limited to Atlanta, although McAvoy referred to other "fruhmenschen" cases against black officials in other cities.

McAvoy, also reached in Atlanta, declined to comment. Diader Rosario, an FBI press spokesman in Atlanta, also declined to comment on the affidavit but said the FBI "has no official or unofficial policy regarding anyone, black, Hispanic or anyone else."

Friedman also was recently involved in a $20 million lawsuit against the FBI in which he claimed the FBI failed to protect him in his undercover work when he was injured and lost a leg in a 1982 car bombing allegedly involving organized crime figures. A federal judge ruled the FBI was not responsible. Friedman has appealed the ruling.

The tape recording involving Ramsey's wife, according to Mitchell, originated in June 1985 from covert electronic surveillance by government agents of Quazer International Inc., a Baltimore importing firm, run by convicted heroin smuggler David Richeson.

Mitchell said he had gone to Richeson's office to talk with a local public relations man there about promoting a testimonial for himself at the Baltimore convention center. While there, Mitchell said, he also decided to call Tucky Ramsey because of her influence in local public relations.

Mitchell said he and his lawyers long ago asked prosecutors for this and other tape recordings involving him at Quazer in preparation for his various trials. But he said the prosecutors did not make them available until late last week, on the eve of his present trial. He said parts of his conversation with Tucky Ramsey had been "spliced out," eliminating all references to her last name.

Mitchell said the judge's wife praised him during the conversation as a "great public servant" but said the federal Hatch Act barred her from helping him in any political endeavors. If Mitchell decided to have her testify at his trial about this, he said, it would "create a conflict of interest for the judge," and he should get out of the case.