manifestacio

This is the one that happens in the street. A manifestacio is a public, collective showing of an opinion: a march, a rally, a picket, people with placards. The verb is manifestacii, and a participant is a manifestacianto. If you are describing the news, this is your word.

The pair to keep straight. English packs two quite different things into demonstration, and Esperanto does not. Demonstri at rank 2,833, three words back in this same band, is to show or prove something, in a classroom or a laboratory or a shop. Manifestacio is the political event. They are unrelated in Esperanto and the confusion is purely an English import. Li demonstris la aparaton is he demonstrated the device; li partoprenis manifestacion is he went on a demo. Nothing about the first sentence implies a placard.

The third member of the family is manifesti. That is to make something evident, to manifest it, which is the sense closest to the Latin. A symptom manifestas an illness. A manifesto with no -aci- in the middle is the printed declaration, which is why the Communist one is Manifesto and the march against it would be a manifestacio.

Where the Latin came from. Manifestus meant caught red-handed. The usual account puts manus, the hand, at the front of it, with an old participle meaning struck or grabbed behind. So what is manifest is not merely visible; it is the thing you were holding when they caught you. A crowd in a square is being manifest in exactly that sense.