Islamist hardliner Anjem Choudary told The Associated Press that he and his fellow demonstrators were near Stratford station, the arrival point for spectators going to London's Olympic Stadium.
He said he was protesting what he described as human rights abuses by China, the U.S. and the U.K., the three nations leading the Olympic medal table. Scotland Yard said it was aware of the demonstration and roughly 20 people were involved.
Choudary and his followers routinely organize attention-seeking protests around major events.
Organizers had worried that the London Olympics would be a magnet for demonstrators, but the event has seen few protests.