Speaking by phone from Cairo, Human Rights Watch's Joe Stork told me that he is alarmed by the U.S. media coverage portraying the clashes on the streets as spats between "rival protesters" -- citizens who have two different visions of the future of Egypt: 

"These are not rival factions. This is brown-shirt tactics. This is the government sending in people -- whether they are paid or not is a very subsidiary question -- sending in thugs armed with knives, stones, sticks, to attack the pro-democracy protesters, who were there in an entirely peaceful manner."

Asked how we can be sure that the pro-government crowds had been sent by the government, Stork cited several bits of evidence, having been in Tahrir Square when the fighting erupted this morning: People he spoke to there mentioned young men being paid as much as $500 to fight for the regime; others who were caught looting were later found to have IDs indicating that they were members of the Ministry of the Interior-controlled security service. 

Were this a rival protest, they could easily have gone to one of the many other public squares in Egypt. Instead, the Army began "letting people in [to the square] today who had mayhem on their minds." "Any one of these things is circumstantial," he explained, "but altogether" the conclusion is clear.