As Cecile Edel, one of the leaders of the Marche Pour le Respect de la Vie said (via google translate):
"the respect for life and respect for the family are part of a single struggle: a struggle eschatological the battle of David against Goliath, of good against evil and fight for the values that we intend to defend. This is indeed the struggle for life in general."
Amen! Estimates of the crowd size range from 350,000 to 800,000, but this video shows people as far as the eye can see in either direction.
Good crowd size shot at the end of this video, facing the Arc de Triomphe:
And the end of this one shows enormous crowds of people pouring into what is, I guess, a park by the Eiffel Tower.