An alleged co-conspirator at the Abscam bribery trial of Sen. Harrison A. Williams Jr. (D-N.J.) testified today that he "begged and pleaded" with the senator not to attend a Jan. 15, 1981, meeting with an Arab sheik because he feared the sheik and his associates could be "con men, gangsters or government agents."

Henry A. (Sandy) Williams III, not related to the senator but a longtime friend and business associate, also testified that for years, before he ever met the Abscam agents, Sen. Williams had used his influence to help other business ventures in which he held a hidden interest.

In testimony that seems to erode further the defense's contention that the senator was fooled into becoming a party to the meetings with the sheik and his associates, Sandy Williams said the senator received undisclosed shares in several firms in exchange for his help in arranging meetings with local and state officials to help the companies.

Sandy Williams also held an interest in the firms, which were to recycle garbage and scrap plastic -- and later to mine titanium -- but he later acknowledged under defense cross-examination that the shares were worthless.

At the secretly videotaped Jan. 15 meeting, the senator promised an FBI agent posing as the sheik that he would help him immigrate to the United States. The tape shows Williams turning down a bribe offer at the meeting, saying his interest was in a titanium mine, in which he would hold a hidden interest.

Sandy Williams said the senator attended the meeting despite his warning. He said the senator told him afterward that when the "sheik" left the room, he opened a drawer and "I never saw so much money in my life." That incident was not on the videotape shown to the jury last week because the tape was shut off when the "sheik" left the room.

Sandy Williams, a 46-year-old unemployed businessman, testified as a government witness under a grant of immunity from prosecution. He said the senator also told him after the meeting "they offered me $50,000 cash and I told them I wasn't interested in cash. I was interested in the titanium mine."

Sandy Williams is considered a crucial part of the government case because his testimony covered numerous meetings of the alleged conspirators that could not be taped by FBI agents.

The senator and codefendant Alexander Feinberg, his longtime personal attorney, are charged with bribery and conspiracy in connection with a scheme to trade Williams' influence in getting government contracts for titanium in return for a hidden share in the venture.

Williams, 61, was chairman of the Senate Labor and Human Resources Committee until Republicans took control of the Senate in January. He is the only senator indicted in the Abscam investigation, which has resulted in the conviction of six House members.

Sandy Williams said he participated in the plan to get a $100 million loan from the "sheik," but became disenchanted in the fall of 1979 because he felt the sheik's representatives were stalling.

When he heard the sheik wanted a January meeting with the senator to ask "a favor," he issued his warning. He said he told the senator he thought the associates -- actually undercover FBI agent Anthony Amoroso and informant Melvin Weinberg -- were phonies who were "leading us on."

Later he told the senator "they were dangerous, that he shouldn't have anything to do with these people. I pleaded with him not to go, I begged him not to go."

Under questioning by prosecutor Edward A. McDonald, Sandy Williams also testified that the senator rejected plans to accept $20,000 cash in "expense money" from Amoroso and Weinberg only because he didn't want to handle the money himself.

At one point, Sandy Williams testified, the senator told him they should take the money because "maybe that's the Arab way; if we don't they may be insulted." The senator discussed investing the money in trotting horses and buying a car for his son before finally calling off the proposed deal.