Professing shock and dismay, House and Senate Democratic leaders yesterday promised thorough investigations of bribery allegations involving eight members of Congress targeted by the FBI in an unprecedented undercover investigation of official corruption.

House Speaker Thomas P. O'Neill Jr. (D-Mass.) said he was determined to "let the cards fall where they may" and voiced hopes that the controversy could be cleared up as quickly as possible.

"The institution has been hurt," O'Neill told reporters at a news conference. He called the allegations "extremely serious" and said he expected a comprehensive and impartial investigation by both Congress and the Justice Department.

In the Senate, Majority Leader Robert C. Byrd (D-W.Va.) said the inquiry there would be bipartisan. He said he was "very disappointed . . . discouraged [and] shocked" at the reports.

Members of the House and Senate ethics committees took initial steps to launch their own inquiries, but they acknowledged that their progress hinges on the willingness of the Justice Department to share its evidence with them.

Undercover FBI agents, posing as Arab sheiks or their representatives, made videotapes of some lawmakers taking cash and others discussing possible favors at meetings in New York and Washington during the 18-month probe.

Those under investigation include Sen. Harrison A. Williams Jr. (D-N.J.) and seven members of the House, Reps. Frank Thompson (D-M.J.), John M. Murphy (D N.Y.), John W. Jenrette Jr. (D-S.C.), John P. Murtha, Raymond F. Lederer and Michael Myers, all Pennsylvania Democrats, and Richard Kelly (R-Fla.).

No charges have been filed yet; the government expects this week to start presenting its evidence to grand juries in four cities.

A Justice Department spokesman said last night that "we have cooperated with congressional investigations in the past," but he emphasized that "we have not provided information in open cases."

He noted that in the past, such as in the South Korean influence-buying scandal, Justice Department officials have provided briefings to lawmakers, but the lawmakers have had to conduct their own investigations and compile their own evidence.

Sen. Howell T. Heflin (D-Ala.), the chairman of the Senate Ethics Committee, said this may prove impossible. "They've got all the information," he said of the FBI and the Justice Department. "We have none."

For its part, the Justice Department announced only that it is starting "an intensive investigation" of how news of the FBI "sting" operation, code-named Operation ABSCAM for "Arab scam," leaked out over the weekend.

"The disclosures made by the media are regrettable because they may injure the reputation of innocent people," Robert M. Smith, special assistant to Attorney General Benjamin R. Civiletti, said in a prepared statement. He said the department intends to find out "whether deliberate disclosures have been made by federal employes."

Smith said later yesterday, in answer to a question, that Civiletti did not tell President Carter about the congressional targets of the "sting" investigation or the impending weekend stories.

Other members of Congress were approached during the course of the FBI investigation, but said they rejected the overtures. Three Democrats from New Jersey reported yesterday that they, too, had been contacted by individuals claiming to represent Arab sheiks who were interested in investing money in the state.

The three are James J. Florio of Camden, William J. Hughes of Ocean City and James J. Howard of Spring Lake Heights. Florio and Hughes said they were apprroached by a man who identified himself as Joseph Silvestrri. Howard would not say who contacted him.

Silvestri reportedly works in the Atlantic City construction industry. Florio told The Washington Post that Silvestri "was in my office on Nov. 7, telling me he'd like me to meet his Arab friends who had more money than they knew what to do with.

"Silvestri said the generosity of his Arab friends was so great that it could easily lead to offers of $25,000 just for making contact with them." Florio recalled. "I declined to do so."

Florio said Silvestri called him later that day and asked him to reconsider. "He said he was with his friends at the Four Seasons Hotel in Georgetown and they were having a good time," the New Jersey Democrat recalled. "He was loud, boisterous and at the time, I thought, obnoxious. I now think he was play-acting, looking for responses and probably wired for the FBI."

If the investigation should result in any indictments, defense lawyers will doubtlessly maintain that their clients were entrapped, that they had "no predisposition" to commit any crime until the FBI planted the idea in their heads.

"That'll be the battleground," one source said. "It's going to be a donnybrook."

Sources said the government anticipated this and that no FBI agent suggested in the first instance that the public officials do anything wrong. Instead, it was said, their own friends and associates made the suggestions. "They'll say that's as good as an FBI agent [doing it]," a source predicted. "It's going to be fascinating."

Rep. Hughes said Silvestri contacted someone in his office, around the same time of the Florio meeting, to try to arrange an interview with Hughes. Hughes said the meeting never materialized.

In a statement, Howard said he called the FBI to his office yesterday to report "what was apparently an effort a few months ago by the so-called Arab sheiks to set up a meeting with me in regard to investing money in New Jersey." He said "the possibility of a contribution following such a meeting was mentioned by those relating the request," but Howard said he flatly turned down the request.

"That was the end of it," he said. "There was no meeting and I never heard from anyone again regarding the matter."

Several of the lawmakers named over the weekend issued terse statements denying any wrongdoing, but declined to comment further.

Looking haggard and red-eyed, Thompson, the chairman of the House Administration Committee, handed out a release to reporters in a corridor of the Rayburn Building, but said he had had no chance to talk to his attorney and could answer no questions.

Thompson is alleged to have agreed to introduce a special immigration bill for one of the "Arabs" in return for a large cash payoff.

Thompson said he had "violated no law. Nor have I violated the trust placed in me by my constituents and colleagues." He said he was confident he would be vindicated by a full inquiry, but "until such inquiry occurs, in the proper place by the proper authorities, I will have no further comment."

Kelly called a full-dress new conference "to answer any questions the press might have," but prefaced it by saying that "any conversation on the technical aspects of the investigation would be inappropriate."

Kelly added that I have not been involved in any criminal activity," and then, before 11 television cameras and a roomful of reporters, spent the next half hour fending off one question after another, especially about whether he had been videotaped stuffing cash into his jacket and trouser pockets during the FBI operation. He said he regarded such queries as touching on "the technical aspects of the investigation."

The session took on comic-opera overtones as Kelly proclaimed himself still a loyal "fan of the FBI" who wants to get "the bleeding hearts" off the bureau's back. At another point, Kelly declared that "since I have all this attention, I would judge I ought to run for president. I must have done something good."

Rep. Murphy issued a statement saying that he had been approached "by some persons on behalf of Middle East interests" but "a routine background check disclosed to me that these persons were fraudulent."

"These events took place within recent weeks," Murphy continued, "including my determination of the fraudulent nature of the persons involved in this enterprise." He said he disengaged from further talks with these "bogus persons . . . several weeks ago."

According to sources, Murphy was filmed last October at a meeting in New York with Howard Criden, a Philadelphia lawyer, and FBI undercover agents for a discussion of how two "clients" of the agents could get into the country. At the end of the conversation, a closed briefcase with $50,000 in it was reportedly offered to the congressman, but, sources said, Murphy declared, "Howard will take that."

Though they were making no public comments, Justice Department officials were meeting yesterday to discuss how to handle presentation of witnesses to the grand juries.

Though final decisions have not been made in all cases, sources said it is likely that a grand jury in Washington will take evidence against Jenrette and Kelly, and perhaps also against Thompson and Murtha, because all reportedly were filmed here.

A grand jury in Philadelphia is expected to hear the cases against several local officials there, and one in Brooklyn is scheduled to hear evidence against -- and view tapes of -- Williams and Murphy, and perhaps Meyers and Lederer, because the alleged acceptance of cash or stock took place in that district.

Another grand jury in Newark will hear evidence about the involvement of Camden, N.J., Mayor Angelo Errichetti and a member of the state gambling commission. The commission member resigned abruptly yesterday, after reports he took a $100,000 bribe to help get a "client" of the undercover agents a gambling license.

In other developments connected with the "sting" cases yesterday, the New York Times reported that Robert del Tufo, the U.S. attorney for New Jersey, recommended that there wasn't enough evidence to prosecute Williams.

As New Jersey's senior senator, Williams recommended del Tufo for his job.

And in Brooklyn, an investigator for the Immigration and Naturalization Service was arraigned on conspiracy and bribery charges in a spinoff of the same "sting" operation.

The federal complaint said Al Alexandro accepted $2,000 last August at a Kennedy Airport hotel as partial payment for a promise to obtain permanent U.S. resident status for a foreigner.

At a special meeting of the House ethics committee yesterday afternoon, chairman Charles E. Bennett (D-Fla.) said he had already entered into discussions to retain a special counsel for the House inquiry and the committee reportedly agreed in executive session to hire one.

At one point during the open segment of the meeting, Rep. F. James Sensenbrenner Jr. (R-Wis.) moved to subpoena all the Justice Department evidence, including the videotapes, before they were turned over to grand juries. Sensenbrenner, however, agreed to withdraw the motion until the committee hears from the department on whether that might jeopardize possible prosecutions.

For the moment, Republicans dominate the committee, 6 to 4. The House committee usually has six members from each party, but Rep. Morgan F. Murphy (D-Ill.) resigned in December and Murtha stepped aside yesterday. House Democratic leaders indicated that two new appointments will be made shortly.

Rep. John M. Slack (D-W.Va.) said he thought it urgent for the ethics committee to get started before other House committees get into the act.

If that happens, Slack warned, "We're going to subvert the entire process and end up looking like monkeys."