It could not immediately be determined how many hikers were in the area or whether any were missing, said Walter Milan, a spokesperson for the national Alpine rescue corps as he confirmed six people had been killed.
Rescuers were checking licence plates in the carpark as part of checks to determine how many people might be unaccounted for, a process that could take hours, Mr Milan said.
Nationalities or ages of the dead were not immediately available.
Of the eight hospitalised survivors, two were in grave condition, emergency dispatch services said.
The National Alpine and Cave Rescue Corps tweeted that the search of the involved area of Marmolada peak involved at least five helicopters and rescue dogs.
"There are five dead among the people hit by the detachment of the serac," the emergency service's initial tweet said, referring to a technical term for a pinnacle of a glacier. "There are eight injured, two of them in grave condition."
The emergency dispatch service, which is based in the nearby Veneto region, said 18 people who were above the area where the ice struck will be evacuated by the Alpine rescue corps.
The dispatch service said the avalanche consisted of a "pouring down of snow, ice and rock".
Marmolada, towering about about 11,000 feet, is the highest peak in the eastern Dolomites.
"A breaking away of rock provoked the opening of a crevasse on the glacier, leaving about 15 people involved," the emergency dispatchers tweeted.
The Alpine rescue service said in a tweet that the segment broke off near Punta Rocca (Rock Point), "along the itinerary normally used to reach the peak".
It was not immediately clear what caused the section of ice to break away and rush down the peak's slope.
But the intense heatwave gripping Italy since late June could be a factor, Mr Milan said.
"The heat is unusual," he said, noting that temperatures in recent days on the peak had topped 10C. "That's extreme heat. Clearly it's something abnormal."
The injured were flown to several hospitals in the regions of Trentino-Alto Adige and Veneto, according to rescue services.