Thanks a lot for your kind comments, Oscar! Lady Luck was on my side that day, Oscar. First I metered, and given the light conditions, I had a nice EV: f16 at 1/60 is good enough of a quick shot. Then I prefocused the lens as I was walking, and when I saw these two people coming my way, I simply pushed the shutter release. I really didn't think I had caught them (the two following shots were duds), so I was really surprised the trick worked.
There's a website belonging to a Swedish photographer, who using a Leica, prefocuses at 2.5 meters and shoots around. He says he burned more film than he cares to remember before getting his "technique" down pat. Some of his shots aren't too engaging, although he has incredibly expressive photos of mothers in grocery stores, dealing with kids. If I have it bookmarked, I'll post the link here later.
Again, thanks for your comments!