Federal agents culminating an 18-month undercover investigation, "Operation Swordfish," swooped down on a major drug-smuggling and money-laundering ring here today soon after Attorney General William French Smith announced in Washington that the elaborate scam was already completed.

The probe's indictments targeted four bankers, three attorneys and a doctor, along with dozens of residents of Colombia and Miami in what U.S. Attorney Stanley Marcus said was an attempt to "penetrate significant narcotics organizations" as part of the administration's war on Miami-based drug operations. Arrests continued into the night.

Sources in the Drug Enforcement Administration and the FBI indicated that the attorney general's remarks in Washington, made to a group of reporters over breakfast, caught field agents by surprise, coming before the arrests had begun.

Smith's announcement came one day after President Reagan disclosed plans to seek up to $200 million for special anti-drug and crime task forces in 12 U.S. cities modeled after the South Florida Task Force that has been operating here since March under the aegis of Vice President Bush. The announcement came less than three weeks before the congressional elections.

Peter Gruden, who heads the extensive DEA operation for south Florida, said Smith's early morning announcement had "absolutely not" affected the number arrested today from among 67 Americans, Colombians and others named by federal grand juries in 15 separate indictments.

Gruden said 36 persons had been arrested by late tonight in the continuing sweep. In addition, more than $1 million in cash, bank accounts, vehicles and real estate had been seized, along with 500,000 Quaalude pills, 96 kilograms of cocaine and a significant amount of marijuana, he said.

The message to drug smugglers and unscrupulous bankers "is that we have an enforcement program under way that will check these kinds of activities," Gruden said, adding later: "But you have to remember that this is only one operation. No single operation can stop the drug traffic all at once."

Among those indicted but not yet arrested tonight was Marlene Navarro, 33, a Miami resident identified in the indictments as the ringleader of the multimillion-dollar smuggling operation.

Two bankers -- Manuel Sanchez, vice president of Intercontinental Bank, and Lionel Paytubi, a former vice president of Greater American Bank -- were arrested in Miami in connection with what was alleged to be a major money-laundering operation designed to recycle the drug proceeds to make them appear legitimate without leaving a "paper trail" that investigators could follow, Gruden said.

Both were charged with referring clients for money-laundering to DEA undercover agents in return for commissions, and with helping transfer drug money to two financial institutions in Panama, the First Interamericas Bank and the Banco Nacional de Paris.

No banks were charged with illegal activity in the indictments, Gruden said.

The multiple indictments made the investigation one of the most fruitful ever organized in what federal officials acknowledge is a difficult struggle to reduce the flow of illegal drugs through south Florida into major U.S. cities.

They are the first in what investigators have pledged will be a string of indictments aimed at high-level drug smugglers and money movers this fall.

A major goal of investigators is to get inside smuggling and money-handling operations to enable them to win indictments against higher ranking criminals than in the past. These would include bankers who wink at laundering schemes and Colombian drug dealers who operate Mafia-like drug growing and smuggling businesses out of reach of U.S. law.

This goal has proved extremely difficult to reach, however, because many suspects never leave their foreign homes and money handlers here often manage to circulate large sums of cash without technically violating the law.

For example, an FBI-run investigation called "Bancoshares" recently indicted a Miami banker, Orlando Arrebola, on charges he helped organize a laundering operation through the Continental National Bank here. But Arrebola was acquitted by a jury 10 days ago after his lawyer argued that he was unaware the money involved could have been connected to illegal drugs.

In today's indictments, 12 people were listed as living in Colombia, a 13th in Madrid and 18 others in unknown locations presumed to be in South America.

Miami has become the country's major junction for illegal drugs, such as cocaine from Colombia and marijuana from South America and the Caribbean, because of its shoreline, proximity to the sources and large Latin population from whom it is difficult for agents to distinguish growers and runners, investigators say.

Against that background, Reagan named Bush to head the South Florida Task Force in what was depicted early this year as an intensive, but temporary, attack on the traffickers. Reagan's announcement Thursday seemed to indicate, however, that the special effort will be prolonged if Congress approves renewed funding.

"Operation Swordfish," though, was launched by the DEA before the task force came into being. DEA agents set up a bogus investment counseling firm called Dean International Investment Inc. through which the attorney general said $19 million passed that led to 42 U.S. and five foreign bank accounts handling drug money.

This amount, while it seems large, is relatively small by south Florida drug money standards.

Agents in another investigation, "Operation Greenback," have charged a Colombian money changer with funneling $242 million through a single Miami bank account in an eight-month period last year.