The U.S. criminal indictments unsealed yesterday against Gen. Manuel Antonio Noriega portray the Panamanian leader as a key member of an international drug-trafficking conspiracy that began in 1981 -- a time when the Reagan administration embraced him and dismissed reports of his drug ties.

The unprecedented indictments, returned by federal grand juries in Miami and Tampa, Fla., Thursday and made public yesterday, provoked sharp reactions yesterday in Panama and in Congress.

Noriega, who is accused of violating U.S. racketeering and drug laws, was charged for providing protection and other government services in Panama to international drug traffickers who shipped cocaine and marijuana to the United States through Panama. He also allowed large sums of illicit profits from U.S. drug sales to be laundered through Panamanian banks, the indictments said.

But some administration critics in Congress and elsewhere declared that the indictments suggest that until recently the administration either covered up or overlooked allegations against Noriega.

Administration officials strongly denied these claims. And the Panamanian Embassy in Washington, in a strongly worded statement released yesterday, denounced the indictments and accused the Reagan administration of engaging in a "systematic campaign" to destabilize the Panamanian government. The embassy said "it is dangerous in the extreme to challenge" Panama's patience and could "engender unforeseen reactions" in the country, site of the strategic Panama Canal.

The indictments come at a time when the Reagan administration is pushing for Noriega, Panama's military commander and de facto ruler, to resign and permit civilian democracy to take hold in Panama.

Administration officials insist the indictments are not related to U.S. efforts to oust Noriega. Officials said the criminal investigations of Noriega hardened within recent months because for the first time ever, U.S. law enforcement officials obtained evidence they viewed as credible.

A high-ranking administration official said yesterday that "what really happened here is that the legal process ran its course. We monitored the legal proceedings, but we didn't try to influence them."

Until recently, Noriega, 51, has been viewed by the Reagan administration as an important ally in Latin America and had strong backing from the Central Intelligence Agency and the Pentagon, according to current and former U.S. officials.

Administration officials said the abandonment of suppport for Noriega was largely prompted by violent, anti-Noriega riots in Panama last summer, the growth of internal opposition to him and the continued deterioration of Panama's economy.

The Miami indictment, which alleges that Noriega was the key figure in a broad criminal conspiracy, charged that the specific scheme described in the indictment began in the fall of 1981 and continued through March 1986.

In a statement yesterday, Rep. Charles B. Rangel (D-N.Y.), chairman of the House Select Committee on Narcotics Abuse and Control, said that "the American people have been victimized by a full-blown cover-up of the facts on Noriega . . . . {The indictment} could have been done years ago.

"Apparently before," Rangel said, "Noriega was a useful source of intelligence on Latin America. Now the administration may believe he has outlived his usefulness."

Sen. Jesse Helms (R-N.C.), a Noriega critic, said in a statement that he has warned of Noriega's drug activities for a decade but that some U.S. officials have "supported Noriega for too long."

Deputy State Department press aide Phyllis Oakley said yesterday that the Noriega indictment is aimed at individuals and is not an attack on the Panamanian government.

Miami U.S. Attorney Leon Kellner said yesterday that the indictments "make it clear that no one is above our laws. General Noriega controls Panama . . . . he utilized his position to sell the country of Panama to traffickers. He has control of law enforcement, of customs {and} of immigration."

Chances of Noriega coming to trial in the United States are slim because the Panamanian constitution bars extradition of its citizens.

In Tampa, Noriega was indicted on three felony counts and charged with assisting a U.S.-based marijuana-smuggling operation in return for receiving about $1 million in payoffs. Noriega and an associate, Enrique Pretelt, a Panamanian businessman who was also charged, were accused of assisting an operation led in part by Steven Michael Kalish, a convicted drug smuggler cooperating with the probe.

The Miami indictment is a more detailed and broader case. Noriega is named with 15 others in a 12-count, 30-page indictment that accuses him and others of participating in a criminal enterprise in violation of U.S. racketeering and drug laws.

The charges in the Miami case carry a maximum 145 years in prison and $1.1 million dollars in fines, if Noriega were ever tried and convicted.

The Miami indictment alleges that beginning in October 1982, Noriega offered to provide government protection and other services to the leaders of the notorious "Medellin cartel," a Colombian drug ring investigators say is responsible for most of the cocaine smuggled into the United States.

The cartel is alleged to have paid Noriega more than $5 million in bribes.

In return, the indictment said, Noriega allowed the ring to use Panamanian airstrips to fly cocaine to the United States, sold the group chemicals used to manufacture cocaine that had been seized by the Panamanian military and provided information on U.S. attempts to investigate the operation.

For example, the indictment said, Noriega in 1983 passed word to the cartel to delay a cocaine shipment passing through Panama to the United States because U.S. military exercises were under way in Panama at the time.

In 1984, when the Colombian government began a crackdown on the Medellin cartel, Noriega let leaders of the group take refuge in Panama and run their operation from there, the indictment alleged. That same year, the indictment said, Noriega also let the cartel briefly set up a cocaine manufacturing plant in Panama near the Colombian border.

Noriega has repeatedly denied any role in drug trafficking and has said the indictments are part of a campaign by conservatives such as Helms to discredit him and subvert the Panama Canal treaties. The 1977 treaties transfer control over the canal from the United States to Panama in the year 2000.

Officials have said that Noriega, who was chief of military intelligence before becoming the military commander in August 1983, for years has provided intelligence to both the CIA and Cuba. The CIA, particularly under the late director, William J. Casey, considered Noriega to be an important asset, officials said.

One former top military official said Noriega also served as a key back-channel intermediary between several U.S. administrations and Cuban leader Fidel Castro. The Miami indictment alleges that Castro mediated a dispute between Noriega and the Medellin cartel over drug operations in Panama.

Norman Bailey, an economic specialist who was employed at the National Security Council in 1981-83, said both the Defense Department and the CIA strongly resisted efforts to withdraw U.S. diplomatic support from Noriega. Bailey said that as a participant in an NSC review of drug-money laundering, he saw "incontrovertible" intelligence reports linking Noriega to drug trafficking.

Bailey said that the information may not have held up in a court case, but he and others tried to use it to encourage a change in the U.S. policy on Noriega. "We ran up against a stone wall" at the Defense Department and the CIA, which felt "what we get from him is too valuable to jeopardize," Bailey said.Staff writer Lou Cannon contributed to this report.