I was there lastnight before it began, during and after waiting for my train back oop north. We knew something was going to happen with the amount of riot vans outside and police gathering.
The actual 'demonstration' arrived so quickly and with such sudden noise that the police further down towards the eurostar entrance rushed forward to help the main entrance team, it took them a minute to realise that the group of people whom were rushing towards them from near the entrance (including myself) where actually people trying to get away from the gas canisters being thrown into the main entrance and not protesters.
I must say, the police were very well prepared, calm and did a great job of trying to keep the peace and the people who weren't part of the demonstration safe and clear of everything kicking off.
It was very scary though as an onlooker from inside St Pancras, particularly for families with younger children whom were clearly upset by it all.