The presenter, 71, was signing off from the Friday evening bulletin following another significant day in British politics when he made the comment.
It came as protesters brought Westminster to a standstill on what was supposed to be the day the UK exited the European Union.
Speaking at the end of the programme on College Green, where congregations of anti-Brexit campaigners have spent months shouting during outside broadcasts, Mr Snow said: “It’s been the most extraordinary day. A day which has seen ... I have never seen so many white people in one place, it’s an extraordinary story.
“There are people everywhere, there are crowds everywhere.”
The remarks drew criticism online from some viewers who described Mr Snow’s comments about “white people” as unnecessary.
In a statement, Channel 4 said: “This was an unscripted observation at the very end of a long week of fast-moving Brexit developments.
“Jon has covered major events such as this over a long career and this was a spontaneous comment reflecting his observation that in a London demonstration of that size, ethnic minorities seemed to be significantly under-represented.
“We regret any offence caused by his comment.”
A spokesman for the regulator Ofcom said they would have to wait until Monday to say how many complaints - if any - had been registered regarding the incident.
A spokesman said: “We will assess any complaints we do receive before deciding whether or not to investigate.”