Supermassive black holes are black holes weighting a million to several billion solar masses and are ubiquitous at the center of galaxies. Their formation mechanism is still a puzzle. In a conventional scenario they assemble slowly in a bottom-up fashion, starting from small black hole ``seeds'' originating from the collapse of the first stars that formed in the Universe, which weighted around hundred solar masses. Such ``seeds'' would grow slowly by accreting gas from the surrounding proto-galaxy and via merging with other black hole seeds as protogalaxies merged with one another. However, in the last decade astronomical observations have shown that black holes of a billion solar mass formed in less than a billion years after the Big Bang, too fast for the conventional bottom-up scenario. An alternative model postulates that supermassive black holes arose quickly after the Big Bang because they were formed already big, from the rapid gravitational collapse of a supermassive gas cloud of more than a million solar masses. However, so far it was unclear how such a cloud could be assembled in the first place. We show, using supercomputer simulations, that such super-clouds arise naturally from the collision of the most massive among protogalaxies, only a few hundred million years after the Big Bang. Gravitational torques and shocks cause a runaway loss of angular momentum in the interstellar gas of the colliding galaxies; the gas is accumulated at the very center in a cloud of about a light-year in size, weighting more than a hundred million solar masses. The cloud continues to contract down to the resolution limit of the computations, still in the newtonian regime, under its own gravity. Future calculations extending into the general relativistic regime will explore the final collapse of the cloud into a supermassive black hole.