I was reading a French paper on Sunday which said that the French police estimate was that there were 45,000 people there, and of course the organizers' estimate was a much higher 100,000. So at the very tiniest minimum it's 45,000, and in fact it was probably considerably more than that.
And please remember - this was just in Paris. It's not like some of the demos I went to a few years ago (when I did more of that sort of thing), where people would troop down to London from all over the country to make up a crowd 10,000-strong. No. There were protests in all major towns and about 200,000 people altogether throughout France. The Paris protest was made up 99.99% of Parisians.
I saw 5 policemen the whole time I was there - and those were all together strolling through the Gare du Nord, nowhere near the demo. One report in the paper said there had been 2,000 police on standby. I didn't actually see any though. If you don't count the possibility of people bouncing up and down on top of bus shelters causing criminal damage to said shelters, there was no trouble at all. Incredible.
I bet there'll be a bit of trouble on Wednesday, though. That's when several pro- and anti- Le Pen/FN marches are going on at the same time, passing within a whisker of each other. Everyone's really hyped up for Wednesday, too. I wouldn't be surprised if there are some serious injuries.
As for Le Pen, he's got a scarily high level of support, even now. He's not going to win, but he might do unhappily well, maybe taking about 25% or thereabouts (or indeed more). It's possible. Remember that 3% of the electorate voted for Bruno Megret, who famously left the Front National because he thought it wasn't right-wing enough.