Twelve alleged leaders of a multimillion-dollar illegal smuggling ring, which may have helped thousands of Mexicans and Guatemalans enter the United States since 1976, were arrested in a series of predawn raids around the country yesterday, federal officials reported.

The ring, which arranged with Mexican recruiters to pick up illegal aliens near the Mexican border and then drive them in vans, mobile homes and, in one case, a rented school bus, to cities throughout the eastern half of the U.S., is one of the largest ever uncovered, officials say.

While the size of the ring's smuggling operation pales against the one million arrests made each year by the U.S. Border Patrol along the Mexican-American frontier, officials nonetheless called the breaking of the ring a significant step in their war against the flood of illegal immigration. There already are an estimated five million illegal aliens currently living in the United States, and thousands more enter each year.

"Most of those apprehended along the border cross on their own," said David Crosland, acting commissioner, of the Immigration and Naturalization SERVICE (INS). "But a lot come in through mom-and-pop [smuggling] operations. This case will help discourage a lot of that."

INS officials indentified the alleged ring leader as Alfonso Interial, 48, of Galesburg, Ill., a Mexican national who became an American citizen in 1966. Interial, who reported an income last year of about $12,000 as a painting contractor, may have grossed as much as $1 million a year from the operation, said Humberto Moreno, INS' anti-smuggling director in Washington.

Altogether, 17 men were named in sealed indictments returned Monday by a Springfield, Ill., grand jury probing the scheme. The identities of two of those indicted have not been released. Two others remain at large and one suspect is currently serving a prison term in an unrelated case, said Brian Perryman, the INS field agent who coordinated the investigation.

"It's certainly one of the largest, well-organized operations we've come across," Crosland said. He added that the INS has been focusing for about two years on stopping such smuggling operations and that the Internal investigation has "been one of our major efforts."

According to Perryman, the ring arranged with recruiters in Allende, Mexico, to pick up from 100 to 200 illegal aliens each week for the last five years from various sites near Eagle Pass, Tex. The recruiters charged each alien about $80 to guide them across the border to the prearranged rendezvous, with most crossings taking place at night, he said.

The aliens then paid from $300 to $800 each to be driven to such places as Dallas, Chicago and Newark, as well as cities in Oklahoma, Michigan and Florida, Perryman said. As many as 50 to 75 different vehicles may have been involved, with six of them usually on the road each week, he said.

A typical van or mobile home load of about 15 illegals bound for Chicago would net the alleged smugglers at least $4,500, he said. Of that amount, the driver would get $1,500, while Interial is alleged to have received $3,000.

"Our investigation goes back to May 9, 1976." said Perryman. "During that time [the ring] was probably bringing in about 5,000 [aliens] a year."

INS agents became aware last December that a massive operation was involved when "we began to see a pattern to people whom we arrested hauling illegal aliens bound for Chicago," Perryman said. "These aliens were traveling on Interstate 80, a major cross-country alien-smuggling route.

"We began to see that a number of defendants in these cases [previously thought to be unrelated] were all from the same area of Illinois," he said.

Because of possible income tax and labor law violations, both the Internal Revenue Service and the U.S. Department of Labor were called in to aid the investigation, Moreno said.

INS officials say it is virtually impossible to curb the influx of such aliens since the long, meandering border separating the United States and Mexico is largely unihabited, desolate and easily crossed at almost any point.

Compounding the INS enforcement problem, these officials say, is a shortage of manpower and funding. At one point last year, border patrolmen were restricted in how much driving they could do because the INS could not afford to pay for their gasoline.

INS officials say that in addition to indepth investigations into such rings as the one broken up yesterday, they have begun confiscating vehicles used by smugglers. They hope that both tactics will serve as a deterrent. In the 19 months since the confiscation of cars began, some 3,700 have been seized, Moreno said.