PHOENIX, July 2— With residential streets cordoned off for fear of an accidental explosion, Federal agents today seized about 90 high-powered rifles and hundreds of pounds of a bomb-making compound from the shabby bungalow of a man whom officials identified as the ordnance specialist of a local paramilitary group.

The seizures followed the arrest on Monday of that suspect, Gary C. Bauer, and 11 other people on charges of conspiring to blow up Government buildings in Phoenix.

Janet A. Napolitano, the United States Attorney for Arizona, said today that the dozen people under arrest were the entire membership of the paramilitary cell, the Viper Militia.

Proceeding gingerly, even stopping work once so that the local bomb squad could be consulted, the Federal agents removed from Mr. Bauer's house, in the city's northern reaches, almost a ton of ammonium nitrate, about half the amount used to build the bomb that destroyed the Federal Building in Oklahoma City last year and took 168 lives. Law-enforcement officials exploded much of the confiscated compound at an abandoned Air Force base in the desert west of here.

"Had it gone off in the city, the destruction would have been phenomenal," said Joseph Roy Jr., director of the Militia Task Force, an organization that tracks paramilitary activities around the country from its base in Alabama. "This is one of the largest hits ever on an underground organized cell."

But the ammonium nitrate and dozens of rifles were not all that was seized. From Mr. Bauer's house the agents also took away a truckload of ammunition, and from the homes of half a dozen other suspects they seized blasting cord, blasting caps, gas masks, bulletproof vests and camouflage uniforms with the insignia of a viper.

An indictment of the suspects, returned by a Federal grand jury here last Thursday but not disclosed until all were in custody on Monday, alleges four counts of unlawful possession of automatic weapons and three of conspiracy. In one, six of the defendants are charged with conspiracy to furnish instructions in the use of explosive devices as a way of promoting civil disorder.

Ms. Napolitano, the United States Attorney, said the Viper Militia had no known connection to two recent terrorist attacks in the region: the Oklahoma City bombing 15 months ago and the derailment by sabotage of an Amtrak train outside Phoenix last October.

Nonetheless the discovery of the large cache of explosives and weapons unsettled this city.

"We're scared to go to work," Barbara Washington, manager of an office supply shop, said as she looked across the street at the marble-adorned Federal Building, one of seven Federal, state and local government buildings that prosecutors say were identified on a Viper Militia videotape as potential targets. "You don't know who your neighbor is."

In the suburban community of Peoria, where two of the suspects live, Becky Nelson gazed across West Shangri-La Road at a white bungalow where, Federal prosecutors say, Dean C. Pleasant, one of those now jailed, was the host at weekly meetings of the Viper Militia.

"I've never left my children with a baby sitter who wasn't a member of our family," Mrs. Nelson said as her two small sons played at her feet. "I don't even send them to school -- I teach them at home. But now I have to be afraid of the neighbor next door, that he might blow us off the face of the earth."

Reflections on what might have been were evident too in Washington, where President Clinton saluted "the law-enforcement officers who made the arrests in Arizona yesterday to avert a terrible terrorist attack."

"Their dedication and hard work over the last six months," the President said, "may have saved many lives."

Although the Viper Militia had apparently been training with explosives and firearms for two years, it came to the attention of Federal agents only last November, when a deer hunter complained that a heavily armed band of men dressed in camouflage had threatened him and a group of Boy Scouts in a remote section of Tonto National Park, about 100 miles northeast of Phoenix.

The 12 suspects are to appear next in public at a detention hearing in Federal District Court here on Friday. Other than Mr. Bauer, 50, and Mr. Pleasant, 27, they are Randy L. Nelson, 32; Finis H. Walker, 41; David W. Belliveau and his wife, Ellen, both 27; Charles F. Knight, 47; Scott J. Shero, 30; Walter E. Sanville, 37; Henry A. Overturf, 37; Donna S. Williams, 44, and Christopher A. Floyd, 21. All are residents of Phoenix or the suburban towns of Peoria, Glendale and Scottsdale.

In an affidavit filed with the court, Jose T. Wall, an agent of the Bureau of Alcohol, Tobacco and Firearms, painted a picture of the Viper Militia as a tightknit group obsessed with bombs and guns. Mr. Wall said one member, Mr. Nelson, lovingly referred to his Browning automatic weapon as Shirley.

Members swore an oath to "enter into mortal combat against enemies of the U.S. Constitution" and to kill anyone trying to infiltrate the group, said Mr. Wall, whose affidavit was based largely on the account of an undercover Arizona law-enforcement officer, still unidentified, who did succeed in penetrating the group last December.

To weed out informers, one member of the cell, Mrs. Belliveau, who worked for a long-distance telephone company, scanned phone records of prospective members for calls to law-enforcement agencies, the affidavit said.

Despite this vigilance, it said, the undercover officer was able to join the group and to record, on either videotape or audiotape, about 20 meetings, including a weekend session in which explosives and illegal automatic weapons were tested around an abandoned mine in cactus desert 50 miles northwest of here.

Traveling on such outings, their cars loaded with illegal weapons and explosives, the members would take care to inspect their vehicles for broken taillights or license-plate irregularities, so as to avoid attracting the attention of the police, the affidavit said.

Dedicated to resisting the "new world order," the Viper Militia this spring started preparing a computer database of the home addresses of local and national law-enforcement officials, the affidavit charged. Two years ago, it said, three members of the group -- Mr. Pleasant and the Belliveaus -- videotaped seven Federal, state and local government buildings here.

"The video," the indictment says, "contains instructions for the placement of explosive devices, illegal entry, control and destruction of the targeted buildings."

Today, on the wall of the immaculate garden apartment where the Belliveaus live in Glendale, hung a sign that said, "I love my country, but I fear my government." A huge safe, five feet tall and perhaps two feet wide, sat just inside the front door.

Sherry McEwen, the manager of the apartment complex, Union Hills Estates, said Mr. Belliveau had worked there as a maintenance man for the last three years. And for the last six years, she said, another of those indicted, Mr. Shero, has also worked on the four-man maintenance team and been a resident.

Ms. McEwen said she was stunned by the arrests.

So were other residents. "It was the maintenance man!" said Lisa Babb, a neighbor who has two small children, is six months pregnant and on Monday experienced what she described today as not one close call, but two: Ten minutes after she had left the small office building where she works for a tree surgeon, the authorities raided the yard behind the building and found explosives. The landlord there, Ms. Babb said, is the father of one of the suspects, Mr. Nelson.

In another neighborhood, meanwhile, residents said they had not thought it strange that a camouflage-painted jeep had often been parked in the driveway of Mr. Bauer, an Army veteran who, Agent Wall's affidavit said, was named the Viper Militia's "research and development specialist" in April.

In yet another subdivision, Chris Terracino, interviewed by The Arizona Republic, said he had frequently seen his next-door neighbor, Mr. Walker, wear Viper patches on his clothes. According to the affidavit, Mr. Walker, a father of three, was the "captain" of the group.

The Viper Militia is not without its defenders here, one of whom, Ernest Hancock, said in an interview today that the videotape cited by the Government was purely "educational."

Mr. Hancock, a Libertarian Party candidate for the Arizona House of Representatives, said he was a friend of two of the group's leaders -- Mr. Pleasant and Mr. Walker. "There isn't any criminal activity here," he said. "They aren't going out bombing anything, or planning to do so. Their crime is educating other people.

"It's almost a hobby for these guys. They go out and blow up rocks in the desert and shoot their guns. They have been doing this for years. This is no secret."

Others here did not have so benign a view of the suspects.

"I got the impression he got a couple of screws loose," John Kaites said of a coffee-shop encounter between himself and Mr. Pleasant in 1994, when they were running against each other in a State Senate election that Mr. Kaites ultimately won.

Mr. Pleasant, who was the Libertarian Party candidate, "sat down," Senator Kaites recalled today, "and then pulled from behind his back a .45-caliber handgun and laid it on the table. That began our conversation.

"I stated that in Arizona you need a permit to carry one of those," said Senator Kaites, who describes himself as a Reagan Republican. "He replied that was why he was running against me. I convinced him to put the weapon away so the waitress would come."

Of the 12 suspects, 4 are either members or supporters of the Libertarian Party, which advocates radical reduction in the power of government. The party has tripled its size in Arizona during the last three years, although its statewide membership remains modest, at 19,330.

Libertarians said today that their party required members to pledge not to engage in violence. Speaking from Washington, where the party's national convention opens on Thursday, John A. Buttrick, the Libertarian candidate for governor of Arizona in 1994, said, "If they were national party members, and if they were involved in a plot to blow up somebody or somebody's property, that would be a violation of an oath they took on joining."

Outside Mr. Bauer's raided home here today, Lawrence Robertson, a member of another paramilitary group, the U.S. Constitutional Rangers, talked to reporters and maintained that the Viper Militia was simply an educational group.

"They were teaching people where to hit and when to hit," he said. "That's all."

Photo: Gary C. Bauer, one of 12 Viper Militia suspects, in a photo fromhis Arizona driver's license. (Associated Press)