The roof of Hartford's 3-year-old Civic Center collapsed under heavy rain and snow early today, injuring no one but dealing what Mayor George Athanson called "a serious blow" to hopes for downtown revival.

Athanson said the Civic Center, which cost $70 million, was "the focal point of the new beat of Hartford."

The flat, 1,400-ton roof of the 10,000-seat arena crumpled without warning at 4:19 a.m., just hours after 4,000 basketball fans had left the arena.

Initial assessments were that repairs would take 1 1/2 to two years, and Arthur Lumsden, president of the Greater Hartford Chamber of Commerce, predicted business losses of between $15 million and $20 million a year.

Fifteen persons, most of them security officers guarding the two dozen shops and restaurants in a small adjoining section of the complex, escaped unharmed when the roof pancaked. The shops were not damaged.

But Hartford's investment - psychological as well as financial - in the Civic Center as the focus of downtown renewal fell into jeopardy.

Like civic centers in other cities, Hartford's new home for sports, shopping and conventions was seen as the way out of the urban decay of recent decades.

Its 2 1/2-acre roof was hearlded as an innovation - it stood only on its four corners with no supporting walls between in order to offer unobstructed view from all seats. It was raised 85 feet in the air and put into place in one piece.

Investigators were studying the remains to determine whether the weight of the snow and drenching rain caused the collapse - or if there were other factors.

Mayor Athanson, on the scene shortly after the collapse, said "We raised the question at the time, if there's snow and ice, what would happen. We were assured that everything was all right."

The civic center was designed by Vincent G. Kling and partners of Philadelphia.

"I don't think it was a natural disaster," said the bitter mayor. "It was supposedly built to handle New England weather. I think there was something wrong with how it was constructed. If it happened after 15 years, maybe . . . but not after three years."

At least three other roofs in Connecticut collapsed during the storm, one of them killing a man at a Jewett City factory.

Gov. Ella Grasso said her reaction to the civil center collapse was "tremendous sadness to see this tangled wreck, and fervent thanks to God that no one was inside at the time."

"The state is committed to helping a beleaguered city that has tried so valiantly to make the Civic Center the heart of its rebirth," she said.

Reporters joined Grasso on her tour of the building that began through a door that four hours earlier had led into mid-level section of the arena. Now they peered through twisted steel girders at a gray dawn sky.

The roof lay horribly twisted but intact, crushing the arena's floor and seats. A row of red seats, ripped from the floor, was perched precariously on a set of concrete steps.

Chunks of insulation and pieces of steel from the walls were blown outdoors, blocking streets and hanging in ice-encrusted trees. Police cordoned the area while bulldozers cleared the debris.

The New England Whalers, the World Hockey Association team whose championship banners had hung in the Civic Center, were searching for a temporary home, the team's owner said.

"The initial shock is similar to a town that has a heart attack," said John Gillespie, president of the Luettgen Ltd. department's store in the retail section.