President Carter signaled his strong support yesterday for the widening investigations of corruption in the General Services Administration in what was described as an effort to persuade GSA employes to implicate "bigger fish."

Shortly before he left for Camp David yesterday morning, the President held surprise meetings at the White House with GSA Administrator Jay Solomon and Deputy Attorney General Benjamin R. Civiletti, head of the Justice Department's criminal division, which is coordinating the federal investigations of GSA.

Carter "was concerned about the investigations and wanted to be filled in and express his support," said Solomon, who was summoned to the White House from a hiking vacation in the Shenandoah Mountains.

"We feel when Justice is going after bigger fish and people have to decide whether they're going to talk or stand mute, it's important for the president to demonstrate that the investigation is going forward and has his support," a Carter administration official said of the president's meeting with Civiletti.

"Some people apparently thought that this was all going to go away and they could keep their mouths shut," the official said. "Well, it's not going to go away."

Carter acted shortly before U.S. prosecutors here are scheduled to meet this week with lawyers for several GSA building managers who are accused of accepting bribes from contractors who were paid by GSA for maintenance and repair work on federal buildings that never was performed.

"So far, no one (of the GSA building managers) has talked, but now they're starting to feel the heat and they're starting to feel the heat and they're getting about to cave," one source said. "You have that in every investigation. First they stand firm because of peer pressure, but then they start to think about their families, and they're ready to talk."

Meanwhile, GSA tentatively has identified 11 building managers and assistant managers who will be fired or disciplined on the grounds they were negligent in performing their duties.

"The basis will be that they were grossly negligent in that they signed inspection reports saying that repair or maintenance work has been done, when they knew or should have known it was not," a source said.

Solomon's decision to fire the employes, most of whom are targets of criminal investigations, is a departure from the traditional government policy of not taking such actions until guilty verdicts are returned. Solomon is known to feel, however, that squandering taxpayers' money, apart from any criminal intent, is reason enough for firing.

"On that basis, GSA could fire almost every employe in the agency," one law enforcement source said. "Our biggest problem is that they all routinely violate regulations, and when we call someone on it, they say everyone does it."

In recent weeks, Solomon and Vincent R. Alto, a former Justice Department prosecutor hired by Solomon to supervise internal GSA investigations of corruption, met with Civiletti to ask for even more aggressiveness from the Justice Department.

According to sources, they had become concerned that the U.S. Attorney's office here and the Washington Field Office of the FBI might be falling to expand their investigations of maintenance contracting fraud beyond the one group of federal buildings in Washington that originally was reviewed by GSA.

The prosecutors and FBI agents here have responded by complaining that GSA investigators have not been all that cooperative and they do not want to let GSA know everything being reviewed for fear suspects will learn about it.

"This happens in every major investigation when you have different agencies competing for a share of the action," a law enforcement source said.

In addition to the Washington investigation into GSA building maintenance and repair contracts, the U.S. attorney in Baltimore is investigating GSA supply stores that dispense of fice supplies to workers in most government buildings. Investigators have found that 27 of the 30 stores in the Washington-Baltimore region have been paying for goods never received in return for payoffs to store managers of cash, trips, and color television sets.

Other investigations of GSA corruption have being started in Texas, southern California, Chicago, New York and Denver, according to Alto. The U.S. attorney's office in Boston has established a special session for its investigation of GSA contracts according to sources.

Alto has said that, in terms of money, the GSA scandal is probably the largest in U.S. government history.