Horror footage from the Furnas lake in Capitólio saw men and women scream as a huge flat portion of rock broke off from the canyon wall and toppled onto two leisure boats beneath. One of the boats appears to escape the worst of the fall but is swallowed by broken rock and the resulting wave, while the other seems to have been hit directly and sunk.
One passenger, wearing a life jacket, can be seen jumping from the side of the boat before the impact.
Four boats in the video appear to have escaped the falling canyon wall.
The seven second long footage ends as the camera operator takes cover from the incoming debris.
The collapse reportedly occurred around 11:00am local time (14:00 GMT).
According to Lieutenant Pedro Aihara, three vessels were hit by falling rock.
Out of the 32 injured, nine people have been hospitalised.
Some 23 of the 32 were treated for light injuries while the people injured were treated for broken bones.
Divers and helicopters are reportedly on site searching the water for more survivors.
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A separate video posted by Conexão GeoClima, a Brazilian scientific research organisation, showed tourists basking under the spray from a waterfall just a few minutes before the rocks fell.
It said in a tweet: “Minutes before the accident with a rock fall in Capitolio, Minas Gerais, a head of water was recorded in the Furnas Canyon waterfall. Note the sudden volume in the waterfall.”
The governor of Minas Gerais, Romeu Zema, has reportedly sent messages of solidarity with the victims on social media.