A coalition of national civil liberties groups has strongly criticized a Justice Department proposal to immune federal employees from civil suits charging them with violating constutional rights.

In a letter to Attorney General Griffin B. Bell, representatives of nine organizations said they were "astonished" at the idea. They said it would have and effect opposite to Carter administration promises for "greater accountability of officials and stronger safeguards against abuse."

Bell had outlined the proposed legislation in a Sept. 16 letter to Vice President Mondale, saying it would protect federal employees from having to pay damages in suits arising from performance of their duties.

The bill would substitute the government as a defendant and eliminate the need for hiring private attorneys to defend the employees, Bell said.

Revelations of illegal wiretapping and break-ins by U.S. intelligence agencies in recent years has resulted in a blood of civil damage cases against FBI and CIA officials.

In his proposal, Bell acknowledged that one of the "main purposes of the amendment is to improve employee morale . . ." Acountability would preserved by disciplining employees when necessary, he said.

But members of the American Civil Liberties Union, Common Cause, Americans for Democratic Action and several public interest law groups argued in their letter to Bell that other alternatives should be considered first.

They said the timing of Bell's proposal was "particularly disturbing" because Congress and the courts "are recognizing the need to tighten the system of accountability."

The coalition also said it had "serious constitutional questions" about a section of the bill that would make the new law retroactive to cases pending at the time of its enactment.

Bell's disregard of that point in his earlier letter to Mondale was a "serious omission," the critics wrote.