A Reagan administration task force yesterday formally adopted recommendations to make sweeping changes in the nation's criminal justice system aimed at putting more violent criminals behind bars. Some features of the package face an uncertain reception, however, because of budgetary or political constraints.

In what was termed its key proposal, the eight-member panel called on the federal government to spend $2 billion over the next four years to help states build more prisons.

Illinois Gov. James R. Thompson, a Republican and co-chairman of the group, said the prison money is needed to shore up America's "internal defenses" and urged the administration to back the plan.

White House counselor Edwin Meese III, a strong supporter of law enforcement groups, said last week, however, that major funding for any crime proposals is "very unlikely."

Associate attorney general Rudolph W. Giuliani conceded yesterday he wasn't sure the $2 billion price tag would survive, but added: "There's no doubt that in any funding concept, corrections is the first priority."

The task force, which was appointed four months ago by Attorney General William French Smith, also proposed changes in existing law that already have drawn criticism from groups as diverse as the National Rifle Association and the American Civil Liberties Union.

The NRA has opposed task force suggestions to strengthen gun laws by requiring a waiting period before completing handgun purchases and by requiring that owners report lost or stolen handguns to local authorities. An NRA spokesman called them "warmed over Carter and Kennedy" proposals.

Robert Garrick, a Meese deputy who represented President Reagan at the NRA convention in Denver, said yesterday that he hadn't seen the task force report yet, but doubted Reagan would change his long-held opposition to gun control laws.

Giuliani, however, called the gun proposals "sensible and moderate," and said of the NRA, "No vested interest can have things completely its own way."

ACLU officials have expressed fear that several proposed changes in bail, evidence and sentencing rules to favor law enforcement would undermine the constitutional rights of citizens. The panel recommended denying bail to defendants considered a danger to the community, adding a new jury verdict of "guilty, but mentally ill," allowing prosecutors to use tainted evidence if they had a "good faith belief" it was obtained legally, and limiting attacks on state convictions by those raising constitutional claims in federal courts.

Most of the recommendations have been proposed before in Congress, but both Thompson and Griffin B. Bell, President Carter's first attorney general, said they felt the proposals have a good chance to be adopted by the administration and Congress because the public is fed up with violent crime. "There's a different mood in the country now," Thompson said.

"The American people have finally let it be known that they are tired of having to barricade themselves from criminals," said Bell, the other co-chair.

Bell acknowledged that some of the technical proposals, such as modifying the "exclusionary rule" on evidence, and limiting "habeas corpus" petitions in federal court, affect relatively few criminal cases and are symbolic. The task force felt the changes are needed to restore public confidence that the system works.

Although the package calls for additional resources for law enforcement in several areas, Bell said the task force made no attempt to "quantify the cost" except for the $2 billion for state prisons. The former attorney general made several attempts during the panel's hearings, and again yesterday, to sidetrack proposals that the federal government pay for training or operating state and local police, prosecutors and programs. "That sends the wrong signal to the states," he said.

Bell got his task force colleagues to agree to include in its report a suggestion the administration study the idea of mandatory national service for youths. The hope is that service in the military, hospitals or parks and forests would teach discipline to trouble-prone youngsters.