Sen. Harrison A. William Jr. (D-N.J.) and three associates were indicted yesterday on bribery and conspiracy charges growing out of the FBI's controversial Abscam undercover investigation.

Williams, 61, the chairman of the Senate Labor and Human resources Committee, is not facing reelection Tuesday, but was scheduled to campaign with President Carter in New Jersey last Friday until news accounts of the imminent indictment began circulating.

"I am innocent. I did nothing wrong," Williams said at a news conference immediately after the indictment was announced.

According to the nine-count indictment handed up by a federal grand jury in New York, Williams agreed to seek government contracts to benefit a titanium mine in Virginia in return for a $100 million loan from FBI undercover operatives posing as representatives of a fictitious Arab sheik. As part of the conspiracy, the four defendants would own part of the business and the senator's share was to be hidden, the indictment said.

The FBI secretly videotaped several meetings between Williams and the undercover agents.

The indictment was delayed for a week, according to sources, because one participant in the mining venture, Henry A. Williams III, agreed to testify against the senator in return for immunity. Williams, who is not related to the senator, was named an unindicted co-conspirator.

At his news conference, Williams, accompanied by his attorney, George Koelzer, accused the Justice Department of trying the case by news leaks during the nine months since the Abscam story broke in early February.

The senator said that at on point in his dealings with the undercover agents, "A suggestion was made which was improper. It was immediately rejected." Sources familiar with the investigation said this referred to an instance in January -- just before Abscam ended -- in which an undercover agent offered Williams money to introduce an immigration bill.

The indictment says only that on Jan. 15 Williams agreed to help the sheik get into the United States.

Indicted along with the senator were Alexander Feinberg, his longtime personal lawyer; George Katz, a New Jersey and Florida businessman; and Angelo J. Errichetti, the mayor of Camden, N.J., who already has been convicted of bribery and conspiracy in the trial of expelled Rep. Michael (Ozzie) Myers (D-Pa.).

The Williams indictment is expected to be the last in the Abscam investigation, the most sweeping congressional scandal in memory. Rep. John Jenrette (D-S.C.) also has been convicted of bribery and four other House members still face Abscam trials.

Williams is only the third senator to be indicted in 40 years. Sen. Edward Gurney (R-Fla.) was indicted for bribery in 1974, and was acquitted. Sen. Daniel Brewster (D-Md.) was convicted in 1972 of accepting an illegal gratuity from a mail order house. A new trial was ordered, and in 1975 he pleaded no contest without admitting guilt.

There are several reasons why Williams' case has been delayed for months past the indictment of the House members implicated in Abscam. Justice Department officials have acknowledged, for instance, that the case against him is legally more difficult because no money changed hands, as it allegedly did with the House members.

In addition, some Justice prosecutors are concerned that Melvin Weinberg, the key undercover inforant in the investigation, went too far in priming the senator for meetings with the phony sheik. It has been reported that Robert J. Del Tufo, the U.S. attorney for New Jersey, wrote a memo on the Williams case, saying he felt the government's conduct violated the senator's constitutional rights to due process.

The indictment gives this account of an alleged bribery conspiracy involving the senator and his associates that began in January 1979 and continued until February 1980:

On May 31, 1979, the senator met with his associates and the FBI operatives at the Hotel Pierre in New York, the grand jury alleged, and on June 28, at the Key Bridge Marriott Inn in Arlington, the senator "promised to obtain United States government contracts which would benefit the . . . business enterprise."

On Aug. 5, at the Northwest Airlines terminal at Kennedy Airport in New York, Williams accepted stock certificates in the titanium mining venture. Tapes played at the Myers trial in New York in August showed that the stock was in Feinberg's name. Weinberg told the senator that the stock simply could be endorsed over to him when he left the Senate.

On Sept. 11, at the Hilton Inn at Kennedy, the senator agreed to continue his efforts to obtain contracts if the venture were sold to another group of investors. And on Oct. 7, at the Plaza Hotel in New York, the senator confirmed he would not disclose his interest in the venture.

In addition to the single conspiracy count, the senator is charged with two counts of bribery -- one involving the original "loan" and the other a later sale of the mine -- two counts of accepting an illegal gratuity, two of receiving illegal compensation and two of interstate travel to aid a racketeering enterprise.

Thomas P. Puccio, head of the Justice Department's organized crime strike force in Brooklyn, supervised the FBI in the investigation. The indictment was returned in New York because the alleged bribe -- the stock certificates -- changed hands at Kennedy Airport.