Sen. Harrison A. Williams Jr. (D-N.J.) was convicted tonight on Abscam bribery and conspiracy charges for agreeing to trade his influence for a hidden share of a $100 million loan from an undercover FBI agent posing as an Arab sheik.

Williams, 61, is the first incumbent senator since 1905 to be found guilty of a criminal charge. He is the last congressional defendant to be tried in the Abscam "sting" investigation.

The jury returned the verdict in U.S. District Court in Brooklyn about 9 p.m. after 28 hours of deliberation. Williams and co-defendant Alexander Feinberg were found guilty on all nine counts of the indictment.

Like the six House members convicted in the Abscam case before him, Williams fell victim to his own words on secretly recorded videotapes. The tapes showed Williams discussing with the "sheik" how he could help get government contracts for titanium and how he would hide an 18 percent interest in a titanium mine to be developed with the sheik's loan.

Williams listened stoically to the verdict, his chin resting in his right hand.His wife, Jeanette, unblinking, sat a row behind him as jury foreman Ralph Monaco, a 29-year-old burglar-alarm installer, announced the guilty verdicts.

Williams was convicted of conspiracy and two counts of bribery, accepting an illegal gratuity, conflict of interest and interstate travel in aid of racketeering. Feinberg, a 72-year-old Cherry Hill, N.J., lawyer, was convicted of conspiracy and aiding and abetting the senator on other violations.

The two men face up to 15 years in prison on the bribery convictions.

In brief remarks to reporters after the verdict was announced, Williams said, "I'm very deeply disappointed . . . . In my heart I know I did no wrong, no wrongdoing. That makes this verdict most difficult to bear."

He said he had no intention of resigning from the Senate, where he has served since 1958, and would appeal the verdict on grounds that his constitutional rights were violated by government misconduct.

Chief Abscam prosecutor Thomas Puccio said he wasn't concerned that this jury took longer than those in previous Abscam cases to reach his verdict. "They weighed it very carefully. That was very important to do in the case of a United States senator," Puccio said. "Obviously, the evidence was overwhelming. The verdict was justified."

The Senate Ethics Committee is expected to begin an immediate investigation of Williams' conduct in the Abscam case.

The controversial undercover investigation began as an effort to recover stolen artworks and securities by having FBI agents pose as representatives of a fictitious wealthy sheik. But unsuspecting middlemen soon began telling undercover informer Melvin Weinberg and his partner, FBI agent Anthony Amoroso, that they also could purchase influence from public officials.

Despite an impressive string of convictions, however, the Abscam (short fro Arab scam) undercover concept still faces legal tests.

Williams' case was considered the most complicated of the Abscam prosecutions because of evidence that Weinberg coached the senator on what to say at a key June 28, 1979, meeting with the "sheik," and because Williams refused a cash bribe offer. The defense argued that the government entrapped Williams and Feinberg by putting words in their mouths.

As in the other cases, however, the jury apparently agreed with Puccio that the videotapes were the key evidence. Time and again the prosecutor focused the jury's attention on the several meetings Williams held with the "sheik" and his representative to discuss the titanium mining venture in which the senator accepted stock.

Williams took the witness stand for four days at the end of the month-long trial to try to explain his statements. He said he was just out to help his friends, Feinberg and Henry A. (Sandy) Williams III (no relation to the senator), who testified against him to escape prosecution. The senator admitted that he boasted of his influence on the June 28 tape. But he said his conversations about government contracts were meaningless "baloney."

Puccio ridiculed that in his final argument, repeatedly calling the senator a liar. "What do you do when you're caught red-handed on videotape?" Puccio asked. "Was the baloney in the speech he [the senator] gave the 'sheik'? Or was the baloney coming out of the senator's mouth as he was on the witness stand in this case?"

In March, 1979, Williams flew to Florida and met the "sheik" for the first time on an FBI yacht. Puccio mocked the senator's explanation that he was impressed then with the "sheik's" "soft eyes" and sensitivity, noting that the FBI agent posing as the sheik wore sunglasses and never spoke a word.

Defense attorneys George J. Koelzer and Harry C. Batchelder Jr. contended that their clients were seeking financing for a legitimate mining venture using titanium for paint pigment. Weinberg and Amoroso shifted the conversation to the use of titanium as a strategic metal in planes and submarines and set out to get the senator to say he would use his influence to assure government contracts for the metal, they said. At the time, Williams was a Senate committee chairman.

The defense played for the jury recordings of several incidents that it said demonstrated government manipulation of the transaction. For instance, in a June 7 conversation, Feinberg told Weinberg that the senator planned to disclose his interest in the mine. Weinberg, a convicted con man who earned more than $200,000 for his undercover role, replied: "He can't report . . . If he reports, then he can't do nothing for us."

Just before the June 28 meeting with the "sheik," Weinberg gave the senator such a blatant coaching session that two government prosecutors testified for the defense that the encounter was improper. They criticized Weinberg for putting words in Williams' mouth. According to their testimony, he replied, "If we don't do that, we won't have no cases."

At the June 28 meeting, Williams told the "sheik" he would talk to the president about titanium and said there would be "no problem" getting contracts. He also agreed that his stock in the venture should be hidden in Feinberg's name. Later the same day, Feinberg told Weinberg that the senator was "thrilled to death with himself" about the meeting. This seemed to hurt the defense contention the senator was an unwilling victim of Weinberg's coaching.