The glacier collapsed on the mountain of Marmolada, the highest in the Italian Dolomites, near the hamlet of Punta Rocca, on the route normally taken to reach its summit.
The disaster struck one day after a record-high temperature of 10 degrees Celsius (50 degrees Fahrenheit) was recorded at the glacier’s summit.
“An avalanche of snow, ice and rock hit an access path at a time when there were several roped parties, some of whom were swept away,” emergency services spokeswoman Michela Canova told AFP.
Six people had been confirmed dead and eight were injured, she added.
Two of the injured were taken to hospital in Belluno, another in a more serious condition was taken to Treviso and five to Trento.
“The total number of climbers involved is not yet known,” said Canova.
She did not specify the nationalities of the victims, but Italian media reported that foreign nationals were among them.
Helicopters were scrambled to take part in the rescue and to monitor the situation from the air. Rescuers in the nearby Veneto region of northeast Italy said they had deployed all their Alpine teams, including sniffer dogs.
Italian Prime Minister Mario Draghi expressed his “sincerest condolences” to the victims and their families on Twitter.
Images filmed from a refuge close to the incident show snow and rock hurtling down the mountain’s slopes and causing a thunderous noise.
Other footage shot by tourists on their mobile phones showed the greyish avalanche sweep away everything in its path.
The mountain rescue team released images showing rescuers and helicopters at the scene to take victims from the valley to the village of Canazei.
Their task was made harder because the bodies were trapped under a layer of ice and rock.
A team of psychologists was on hand to support relatives of the victims.