The death toll soared to more than 86 and the injury count to 550 yesterday, including victims in Canada, as rescue workers dug through rubble in this nation's worst tornado disaster in a decade.

As the violent twisters swept through Pennsylvania, Ohio, New York and Ontario Friday night, roofs exploded, buildings and homes disintegrated and automobiles flew through department store windows.

The storm, which started shortly after 5 p.m. and lasted about 4 1/2 hours, spawned several dozen tornadoes that churned across counties near the eastern shore of Lake Erie, randomly devastating rural communities and virtually destroying several small towns.

"When I came into the office, everything was wiped out," said Betty Pomp, owner of an answering service in Niles, Ohio.

Describing the area where the tornado cut a 200-foot swath through the town, killing six persons and leaving others injured and homeless, she said: "There's nothing standing. Everything is completely wiped out . . . . The people are walking around in circles."

Pennsylvania Gov. Richard L. Thornburgh (R) and Ohio Gov. Richard F. Celeste (D) toured the devastated areas yesterday and called out their National Guard units to help with the cleanup and to prevent looting.

As rescue workers scrambled during the night Friday and through most of yesterday to find the dead or missing, stunned residents told stories of heroism and tragedy as the tornadoes suddenly began up-ending earth, people and buildings.

While umpiring a softball game in Mercer County, Pa., David Kostka, 32, saw a funnel-shaped cloud moving toward the children. Kostka grabbed his niece, Crista Warrender, 7, and a boy on the bleachers and rushed them to a nearby ditch.

According to Kostka's uncle, Sideris Lambros of Farrell, Pa., Kostka died after he "jumped on top of them to protect them. He was covering them with his body when he was picked up and carried away. The little girl looked up for her uncle, and he just was gone."

In Beaver Falls, Pa., at least two persons were killed and about 30 injured when a twister ripped off the roof of a department store, leaving only the walls standing and terrified shoppers trapped inside.

Henry Austin of Bayonne, N.J., said he was unloading freight for a department store when he saw the tornado grinding up the earth a few hundred yards away. He leapt into his truck, he told reporters, which quickly flipped onto its side. As he pulled himself out, he saw a hand sticking out of the debris.

Digging frantically, he helped free Frank Gerello, a beer distributor who said: "I prayed while I was in there."

Sandra Stahlsmith, four months pregnant with her sixth child, was less fortunate. Huddled against the fruit cellar wall in her home in Albion, Pa., and cradling her son Luke, 6, in her arms when the tornado struck, Stahlsmith said the house suddenly crashed down around them.

"I felt my little boy being crushed. He took two breaths, and I knew he was dead," she said.

"He died in my arms," she said, fighting back tears. "His head was resting right on my chin. I could smell his hair. I tried so hard to pick up that wall with my back; I couldn't move a muscle."

As Red Cross workers and volunteers spanned out across the 21-county area to provide relief and shelter, Federal Emergency Management Agency officials began to assess the extent of the human and financial damages. The American Red Cross set up 27 emergency shelters housing about 1,000 victims of the storm.

Govs. Thornburgh and Celeste are expected to ask President Reagan to declare the section a disaster area.

The massive storm, the nation's worst since April 1974 when 148 tornadoes left 315 dead in the Midwest and South, hit Pennsylvania the hardest. The death toll there as of late yesterday afternoon was 61.

"This is the worst in Pennsylvania's history," said Darcy Charney, public information officer for the Pennsylvania Emergency Management Agency. She said that in 1944 tornadoes killed 45 persons in the state.

Fred Ostpy, director of the National Severe Storms Forecast Center in Kansas City, and other meteorologists said the storm that triggered the 27 tornadoes logged so far came during the normal season for violent weather.

But Ostpy said few tornado clusters move into the Pennsylvania and Ohio area where this one hit.

Andrew Gregorio, a meteorologist for Compu-Weather, a private forecasting service in New York, told a reporter yesterday that climate conditions in the Lake Erie area created the deadly "tornado cocktail." That is, warm air at the surface and a colder jet stream above created dangerous atmospheric instability.

"It's like a teakettle boiling; the air is very unstable," said Scott Prosise, a forecaster for the U.S. Weather Service in Washington.

As bulldozers and police dogs went through the mountains of debris, rescue officials assessed the damage. Storms killed at least 12 in Ohio. In Ontario at least 13 died and more than 150 were injured.

At a raceway in Barrie, Ont., a twister plowed into the buildings, as people screamed for help and terrified horses ran wild.

"People were crying and screaming," said Patricia Burke of Barrie. "All the buildings were down except the one I was in."

Still, the numbers of dead and injured do not convey the intensity of the blow to the small communities in these rural counties. Atlantic, Pa., with a population of 1,200, reported five dead and every building except the Presbyterian church reduced to rubble. Albion, with 1,500 people, lost nine.

In Tionesta, a tiny resort area on the edge of the Allegheny National Forest, a tourist camp turned into a wasteland as a tornado cut a swath 30 miles long, killing seven persons.

"I spent 20 years in the Army, one year in Vietnam," said Sheriff Harry Tucker. "I haven't seen this much destruction. Maybe it seems more that way because it's home."