The toughest gun-sentencing law in the nation will go into effect Thursday when California imposes a new statute that dramatically increases prison terms for anyone who wields a firearm while committing a crime.

Through a massive advertising campaign initiated in recent days, state officials are warning would-be criminals that the law requires that 10 years be added to the sentence of anyone over 14 years old who simply carries a gun -- loaded or unloaded -- in the commission of a serious crime. It requires that 20 years be amended to a term for firing the gun -- even if the bullet hits no one, and that 25 years to life be mandated for seriously injuring a victim.

Although numerous states in recent years have initiated gun bans and other efforts to curb the use of firearms, no other state has so drastically increased the punishments.

All sentences under the new law, which was signed by Gov. Pete Wilson (R) on Sept. 25 with little fanfare or controversy, will be added to whatever punishment is imposed for the crime that was committed with the gun.

This makes it, in the words of Attorney General Dan Lungren, "quite simply the toughest gun-abuse control measure in the nation."

The additional sentences cannot be suspended, probation cannot be recommended and persons convicted will be required to serve at least 85 percent of the additional prison terms after credit for good behavior is deducted.

The sentencing add-ons will apply to gun possession during the commission of 17 crimes, including robbery, kidnapping, rape and assault with intent to commit a felony.

California law enforcement officials predict that the new "10-20-life" law will be copied elsewhere in the country just as the state's 1994 "three-strikes-and-you're-out" law mandating life sentences for third-time felons has been adopted by about two dozen states in one form or another.

As California attempts to get the word out about the new law, no one is being left out. Each of the more than 156,000 inmates in the state's 33 prisons will be handed a brochure in the weeks ahead warning of the law's provisions.

Criminals outside the prison system will get the message about the new law through advertisements broadcast over 300 television and radio stations statewide, featuring actor Alan Autry, who plays tough cop Bubba Skinner on the "Heat of the Night" television series.

"Do you know a tough guy with a gun? Let him know the law just came to town," Autry says before outlining the new sentence enhancements. "Now, if you're 14 years or older, this law applies to you. Use a gun and you're done . . . 10, 20, life, the law is here."

Critics of the new law say it is a well-intentioned but misguided measure that will simply fuel an explosion in new prison construction without addressing either the proliferation of guns or the root causes of crime.

"These types of excessive laws lead to an enormous waste of government resources without solving anything. The government instead should look more closely at regulating the possession of guns in a much stricter fashion so that the use of guns is no longer a possibility," said Elizabeth Schroeder, associate director of the Southern California branch of the American Civil Liberties Union.

The ACLU opposed the measure when it was enacted almost unanimously by the state legislature, calling it "excessive and gratuitous" because existing law already allows for sentence enhancements of up to 10 years when a person uses a firearm in the commission of a felony.

What's different, however, is that the new law mandates the additional sentences.

And that change, according to proponents, is not only critical but won't necessarily swell the ranks of prisons. "We'd rather fill up the prisons than fill the cemeteries," said Sean Walsh, Wilson's press secretary. "But we also believe that when the word gets out there will be fewer crimes committed with guns. We believe it will be a deterrent."

Prison officials have said the short-term impact on the inmate population will be negligible because criminals would be serving time for the basic crime they committed anyway. They said the longer-term impact of the enhanced sentences will be determined by how effective a deterrent the 10-20-life law becomes.

State officials estimated four years ago that the prison population would soar to 230,000 by the turn of the century because of the three-strikes law and that prisons would run out of beds by 1998. But those predictions proved to be off the mark. Department of Corrections officials now say they will be housing only 181,000 inmates by the year 2000.

Among the reasons given for the lower-than-expected increase were that judges have not imposed three-strike sentences as often as was originally expected, and that felons facing non-three strikes sentences leveled off along with the general decline in the crime rate. Supporters of the 10-20-life law say they expect the same type of flattening out of the crime rate to help reduce the pressure on prisons.

But to be successful, the new law will have to be recognized as a certainty by criminals, said one of the measure's chief proponents, Mike Reynolds, a Fresno anti-crime activist who co-wrote the three-strikes ballot initiative after his 18-year-old daughter, Kimber, was shot to death during a purse-snatching outside a movie theater in 1992.

"It's tough stuff. It's a real deterrent, but you can't bluff these people. You need certainty and realness to make it sink in and cause a real drop in crime," Reynolds said.