Buenos Aires again - riots! (not)

Back to BA again, and there was a national holiday to do with the Mothers of The Plaza de Mayo, who are still demonstrating to get justice for their children who went "missing" under the old regime during the "Dirty War". So there were lots of people in the streets with banners and playing samba. Mostly seemed all cheerful and positive enough. But then we went near the bus station, and there were kids standing around with scarves wrapped round their heads and huge sticks, standing in vaguely menacing groups, and smashing the signs on the subway. We stepped into a shop to look for tourist tat, just as they were literally battening up the hatches, "just in case". Across the road, a mini squad of cops with their own sticks were hanging around with their huge vans with the snow/people scoops on the front, in case any real trouble began...! But in the end everything seemed to go reasonably peacefully as far as I was aware.