What kind of balloons are these?
These are gas balloons. Instead of heating the air to reduce its density and providing the buoyancy, these balloons use helium or hydrogen which is lighter than air. Before lifting off, the balloons are kept on the ground by extra weight hanging on the basket. Usually these bags, clearly visible on the picture, are filled with sand.
During the flights, the gas will slowly dissipate through the fabric of the balloon reducing the buoyancy of the balloon. By throwing more weight overboard, again sand, the balloon can maintain in the air.
Gas balloons have less control in height than hot air balloons. The balloons are more expensive to operate because of the price of helium and to a lesser extent hydrogen.
Although gas ballooning is less popular then hot air ballooning, there are a number of gas balloons operating in Europe and the USA.

Once a year the Gordon Bennett race  is held in which teams compete to fly the longest distance in their gas balloon.

After some more research into the Trophee du ballon libre I found this Polygoon Journaal item of 1976 (in Dutch). It mentions that the balloon is kept afloat by 630 cubic meters hydrogen gas.