Spokesmen from almost every level of the federal bureaucracy went to Congress yesterday to denounce President Carter's plan to overhaul the civil service system as "naive and impractical," "a power grab" and "a conspiracy" to destroy the merit system.

Responding to the federal workers' pleas, members of the House Civil Service Committee said Congress will probably amend Carter's proposals drastically, making it unlikely that any of the legislation will be enacted before Congress adjourns in October.

"I'm skeptical the administration is going to get what they want, or get it very fast," said Rep. Herbert E. Harris (D-Va.), whose Northern Virginia constituency includes a large number of federal workers.

The Carter plan is the product of "a conspiracy by federal personnel officers to scuttle the merit system" and "to get rid of every law that ever stood in their way" when they want to fire or shift an employe, charged James D. Hill, head of the National Federation of Professional Organizations, which represents such federally employed professionals as air traffic controllers, scientists and engineers.

In testimony before the civil service committee, Hill and spokesmen for several federal employes' predicted "cronyism and patronage and empire building" by their managers under the direction of a "personnel czar" in the White House if the Carter plan were enacted.

They called for stronger and broader collective bargaining rights for federal unions as the only sure protection for government employes against abuse of their rights by their bosses.

The Carter plan's "curtailments of basic due process (for federal employes) in performance appraisals and adverse actions (such as firing)" are a "basic denial of constitutional rights of due process as guaranteed under the Fifth Amendment," testified Joseph Cook, Jr., an official of the American Federation of Government Employees local at the Department of Health, Education and Welfare.

Cook cited several cases of recent alleged abuses that he said would become more prevalent under the new plan.

He told of an employe at HEW who identified a mathematical error of $54 million made by her supervisor in the public assistance program. Rather than getting credit for finding the error, the employe was blamed for causing it, Cook testified, and was subsequently harassed and "physically assaulted" by her supervisor. "They (supervisors) have threatened a propose disciplinary action against her in order to prevent her from filing a complaint," Cook told the committee.

The idea that government workers are a "privileged class that is overpaid, underworked and supersecure is a 'big lie,'" the Carter administration is spreading, Cook said.

Officials of the National Association of Supervisors (in the) Federal Government, called the administration's proposal for a senior executive service "nothing new" and said ti would lead to "politicalization" of the government service.

Abolishing automatic pay increases and instituting incentive pay (for good performance) for managers would be "beautiful but impractical," they said, and has not even worked in the private sector, where there is also a profit motive. "Claims and counter-claims" by those denied incentive pay bonuses would place an intolerable burden on the system, they said.

The executive director of the supervisors organization, Bun Bray Jr., also suggested that the Office of Management and Budget be "taken out of the personnel business." Soem Congressmen on the committee indicated agreement. They added that they see OMB as a barrier between themselves and the Civil Service Commission, making it difficult for them sometimes to obtain studies or other information they need in order to make informed recommendations.

"It's like kissing through a picket fence," said Rep. Harris, "I realize an administration needs uniform policy, but we want to be a part of the ball game as policy is being decided, before the compromises have been made and the information filtered through OMB."

While the opponents each found fault with most of the Carter plan, they differed among themselves on some details. They were least opposed to the proposal to split the Civil Service Commission into two separate bodies - one to protect employes interest and one to promote management interests. The CSC now plays both, often conflicting, roles.

The Carter administration has argued that its proposed changes are necessary to make government more effective and responsive to the needs of the taxpayers who support it. Carters legislative package is designed to streamline the hiring and firing processes, give managers greater flexibility in the way then use their work force, and introduce an incentive pay system for good performance. At the same time, the legislation is supposed to strengthen and simplify employe appeal rights and other protections against political abuse, according to administration proponents of the bills.