WINTER PARK, COLO. -- A boulder dislodged by state highway workers rolled off a mountainside yesterday and smashed into a moving sightseeing bus, killing seven people and injuring 15 others, four of them critically, authorities said.

Six of the 28 people aboard, Americans and foreigners on a 250-mile tour through Rocky Mountain National Park, escaped injury. The Gray Line bus was nearing the foot of Berthoud Pass on U.S. 40 about 60 miles west of Denver when the boulder hit at 10:50 a.m.

Colorado Highway Department spokesman Dan Hopkins said that a crew was clearing rock above the roadway and that a front-end loader dislodged one weighing several tons.

Although the worker had pushed the boulder onto a large flat area, thinking it would stay there, the rock went "over the edge of the large flat area down through several hundred feet of trees onto the highway . . . ," he said.

Doug Freed, editor of the weekly Winter Park Manifest, said, "A sheriff on the radio said the right side of the bus is completely gone, just like it was cut open by a can opener."

The most seriously injured passengers were transported by helicopter to Denver-area hospitals, officials said. The other injured people were treated in Winter Park and Granby. The driver was slightly injured.