The newsreel began with the printed words: "We believe it our duty to screen these pictures as a warning to future generations."
What followed left audiences all over Britain in a silence still remembered by those who are old enough.
The three-minute film ended, according to the Pathe News catalogue, with a "panning shot of the dead and charred bodies spread across endless fields. Most are skeletons".
Yesterday this first footage of Belsen concentration camp, screened in cinemas on March 30 1945, went online - with 100,000 other historic stories - for the benefit of new generations who have seen only flickering extracts from them on television.
It is part of an archive of 3,500 hours of news bulletins starting in the era of silent cinema in 1902, when Pathe News opened in Britain, and ending in 1970.
For much of the 20th century, it was the public's main communal source of visual news. It was killed by colour television and by the increasingly out-of-date flavour of deferential pomposity in its news coverage.
But its archive is a priceless asset whose images have been mined by television, films, home video and advertisers. It has 84 clips of the great mid-century footballer Stanley Matthews, the first in 1932.
The twice-weekly bulletins covered events including the Titanic disaster in 1912, the first world war, the 1926 British general strike, the American depression, the atomic bombs which ended the second world war, and the first moon walk.
The footage was digitised as part of a £50m programme under the national lottery new opportunities fund. A mock-up of the Pathe News cockerel, whose image opened each bulletin, walked Trafalgar Square yesterday as the culture secretary, Tessa Jowell, launched the website.
Some 15,000 hits were counted on the site by lunchtime yesterday and 3,500 people downloaded film clips. Clips are free at present but web pages already have forms which would allow prices to be charged. A spokesman for Pathe News, which is owned by the Daily Mail and General Trust Group, said subscriptions would be introduced in several years' time, "but they will not be prohibitively expensive".
He added: "The site will be immensely useful to historians and to others. There are enough footballers on it to keep a fan going for months."
A long-running complaint against Pathe News was that it followed the views of governments too closely in framing news for its mass audiences. A tendency towards propaganda is reflected in the archive.
Clips on the general strike emphasise the reassuring presence of soldiers and volunteer civilian reservists rather than the issues of the strike.
The Hiroshima and Nagasaki newsreel opens with women in Trafalgar Square "barn dancing" with joy at the Japanese surrender. Only later, according to Pathe's catalogue, is it made clear the surrender was caused by "two huge explosions at night".
The archive is at www.britishpathe.com