WASHINGTON (AP) _ Sparks flew, flames roared and furnishings exploded when the Consumer Product Safety Commission on Wednesday demonstrated the dangers of fireworks.

In its own version of ''the big bang'' theory, the agency warned consumers that Fourth of July celebrations could be marred by tragedy if precautions aren't taken to avoid the burns and injuries caused by improper or illegal use of the devices.

''We know that some 10,000 Americans, most of them adolescents and teenagers, will end up in hospital emergency rooms this year from injuries associated with fireworks,'' said CPSC Chairwoman Jacqueline Jones-Smith.

''I can guarantee you that those injuries could have been prevented,'' said Commissioner Anne Graham. She said parents should forego their own fireworks and take children to community fireworks displays by professionals.

The agency estimated 6,300 injuries, including burns, disfigurement, loss of eyes, fingers, hands and other limbs, were associated with fireworks in 1989.

Two-thirds of those injuries occurred between June 23 and July 20, the height of the Independence Day season.

The commission spotlighted five categories of fireworks in its annual demonstration on the mall near the Washington Monument.

The five - firecrackers, rockets, sparklers, reloadable aerial shells and firecracker salutes - cause most of the injuries associated with the devices, said John Rogers, the agency's fireworks expert.

CPSC staffers showed how a common practical joke - putting firecrackers in an unsuspecting individual's pocket - could cause serious burns.

A package of six firecrackers was placed in the pocket of a baseball uniform worn by a doll and ignited. A small smoky explosion seared a hole through the outfit to the doll's skin.

Vincent Baxter, a 12-year-old Maryland boy who suffered third-degree burns in a similar firecracker accident two months ago, said he lighted and tossed a firecracker that ricocheted back at him and set his clothing on fire.

''It caught another firecracker on fire,'' he said. ''Then it started sprinkling flames and sparks everywhere and caught my leg on fire.''

''I just want to say don't play with firecrackers,'' he warned children.

The CPSC demonstrated that said even sparklers, considered among the safest of small fireworks, can also safe by most people, can cause clothing fires.

''People tend to think sparklers are safe, especially for young children,'' said Rogers. But ''giving a sparkler to children is like giving them a lighter or a book of matches to play with.''

Half the fireworks injuries to children up to four years old were caused by sparklers, the commission said.

The commission also said it had taken the initial step toward a possible ban of some reloadable shell aerial fireworks devices.

The devices consist of a tube launcher and shell with a 6- or 7-inch ignition fuse. The long fuse makes users believe they have plenty of time to get out of harm's way, said Rogers, but the fuses can burn in less than a second.

The commission warned that the force of the exploding shell presents another danger. They set off one shell that shot through three plywood boards 3/16th of an inch thick that were lined up five inches apart and 2 1/2 feet from the launcher.

''Fireworks by their very nature are dangerous,'' said the commission. They warned consumers to take the following precautions:

-Check with state and local authorities to find out which fireworks are legal in your area.

-Read the labels for fireworks carefully when you buy them. If they don't have caution labels, don't buy them because they probably don't meet other safety standards.

-Don't use fireworks in crowded areas, near dry grass or combustible material such as gasoline or charcoal lighter fluid.

-Never place your face or other part of your body over fireworks devices and don't try to use them if you have been using alcohol or other drugs.

-Supervise children and never give fireworks, even sparklers, to very young children.

-Avoid firecracker salutes, particularly M-80's and cherry bombs.