RFE/RL reported on their protest here. But it turns out that their detentions were not a simple matter; in fact, they required a little assistance from a pair of agents provocateurs, RFE/RL's Russian Service reports.
Each of the opposition leaders began his protest standing alone. Under Russian law, that’s not a punishable offense; an individual needs no prior permission to hold a sign in a public place. But when the one-man protest turns into a crowd of three, the police can charge the participants with holding an unsanctioned demonstration.
And that’s exactly what happened as a pair of men in hoods joined opposition leader Boris Nemtsov on his solitary protest. After the police detained Nemtsov for leading the mass action, Vladimir Milov stepped up to take his place -- and again, two hooded men boosted the demonstration’s numbers to three. He was also detained, as were two more opposition leaders who began individual protests only to find themselves leading a crowd.
See a slide show of the four consecutive three-man protests.
-- Margot Buff