KANSAS CITY, Mo., Nov. 29— Six firefighters were killed by two powerful explosions at a construction site before dawn today as they battled fires that police officials believe were caused by arsonists.

The force of the explosions, about half an hour apart, was so great that people were jarrred in their beds as far as 10 miles away. Many windows in surrounding neighborhoods were broken.

A seventh firefighter, arriving on the scene as a driver for a battalion chief, was injured when an explosion blew his Fire Department car 25 feet and shattered its windows.

The deaths were the worst Fire Department disaster in the city's history, according to Harold Knabe, a department spokesman. Five firefighters were killed in the worst previous accident, the explosion of a gasoline tank in a 1959 fire. Two Huge Craters

At the scene, only one twisted chassis remained early today as evidence of two fire trucks that arrived with the six firemen before 4 A.M. The wreckage lay near two craters, 30 to 40 feet wide and about seven feet deep.

Later today, after questioning two security guards who had been at the scene, the police put together the following sequence of events:

The guards, stationed there by the Brown Brothers Excavating Company, which is excavating for a new highway, saw someone moving near the edge of the work site and went to investigate.

A fire then broke out in a pickup truck that the guards had parked nearby. The firefighters arriving for that fire then saw fires burning at two trailers that had been left parked at the site. Both trailers contained large amounts of ammonium nitrate, an explosive used for exavation jobs. Details Are Sketchy

The first of the two explosions occurred about 4:10 A.M., the second about a half-hour later. Investigators did not know how many victims had been killed in each blast.

The site is a sparsely populated area on the southwest edge of the city about a quarter-mile from Bannister Mall, one of the city's biggest and busiest shopping centers.

The six who died were two captains, Gerald C. Halloran, 57 years old, and James H. Kilventon Jr., 54, and Thomas M. Fry, 41, Luther E. Hurd, 31, Robert D. McKarnin, 42, and Michael R. Oldham, 32.

Firefighters had been warned to approach with caution for fear of explosives, according to Richard Wright, a department communications official. No Motive Yet Perceived

The police have been unable to suggest a motive for an arsonist. An investigator at the scene said there were no earlier reports of crime there. Officials of two unions that work for Brown Brothers said they knew of no labor problems.

Seven city detectives, aided by agents of the Federal Bureau of Alcohol, Tobacco and Firearms, are investigating.

John Hall, a spokesman for the National Fire Protection Association, a nonprofit safety group based in Quincy, Mass., said the worst fire in modern memory in terms of firefighters' deaths occurred in Sun Ray, Tex., in 1956. Nineteen men died fighting an oil refinery fire.

In Brockton, Mass., in 1946, 13 firefighters died fighting a theater blaze. In 1966, 12 New York City firefighters died at a fire in a commercial building near Madison Square Garden, and 12 firefighters and a civilian were killed in Kingman, Ariz., in 1973.

photo of site of blast in Kansas City, Mo. (AP)