They're often there as themselves also, as opposed to just their various front groups. At a few of these relatively recent events I've seen placards with the "RCP" (Revolutionary Communist Party) logo at the bottom.
For what it's worth, the 1992 LA/Rodney King riots were instigated by the Revolutionary Communist Party...
From David Horowitz's FrontpageMag.com /DiscoverTheNetworks.org:
"Throughout its history, one of RCP's [Revolutionary Communist Party] principal objectives has been to foment civil unrest in the United States. The most notable example of such efforts occurred on April 29, 1992, when RCP members looted and trashed the downtown and government districts of Los Angeles, triggering the infamous Rodney King riots. During the days immediately preceding the violence, RCP -- which maintained close ties to the L.A. gangs known as the Crips and the Bloods -- had circulated throughout South Central Los Angeles a leaflet featuring a statement by RCP National Spokesman Carl Dix, titled 'It's Right To Rebel' -- a quote popularized by Mao Zedong.
Encouraged by Dix, RCP activists helped lead the riots that would leave 58 people dead, more than 2,300 people injured, some 5,300 buildings burned, and $1 billion in property damaged or destroyed. On the ten-year anniversary of the rioting, RCP member Joseph Veale fondly recalled the violence as 'the most beautiful, the most heroic civil action in the history of the United States.'"
http://www.discoverthenetworks.org/groupProfile.asp?grpid=6197
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Here's the original source for the Van Jones (Obama's selection to be his "Green Jobs Czar") quote about the time he spent in jail following the Rodney King/LA riots (Jones was arrested in "peaceful" protests in San Fran). It was in a 2005 interview he did with East Bay Express. They of course (what else?) claim he "renounced his rowdy Black Nationalist ways" since then (yet he's calling for "complete revolution" in Aug 2009! ...):
But in jail, he said, "I met all these young radical people of color -- I mean really radical, communists and anarchists. And it was, like, 'This is what I need to be a part of.'" Although he already had a plane ticket, he decided to stay in San Francisco. "I spent the next ten years of my life working with a lot of those people I met in jail, trying to be a revolutionary." In the months that followed, he let go of any lingering thoughts that he might fit in with the status quo. "I was a rowdy nationalist on April 28th, and then the verdicts came down on April 29th," he said. "By August, I was a communist."
http://www.eastbayexpress.com/gyrobase/the-new-face-of-environmentalism/Content?oid=1079539&showFullText=true
Maxine Waters on the Los Angeles riots of 1992 (Rodney King riots):
"[Maxine] Waters has been criticized for her comments regarding the Los Angeles riots of 1992. In defense of the people that looted stores and damaged property, Waters said 'If you call it a riot it sounds like it was just a bunch of crazy people who went out and did bad things for no reason. I maintain it was somewhat understandable, if not acceptable. So I call it a rebellion.'[5] She also said it was 'a spontaneous reaction to a lot of injustice' and 'The anger in my district is righteous. I'm just as angry as they are.' She responded to the mass looting of Korean-owned stores by saying: 'There were mothers who took this as an opportunity to take some milk, to take some bread, to take some shoes. They are not crooks. Everybody in the street was not a thug or a hood.'"
http://en.wikipedia.org/wiki/Maxine_Waters#Los_Angeles_riots_of_1992
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"The RCP [Revolutionary Communist Party] upheld the 1992 sometimes-violent unrest in Los Angeles and nationally as a 'rebellion' in the aftermath of the Rodney King verdicts. Then-LAPD chief Daryl Gates alleged that the RCP was involved in the 'riots'. Los Angeles has long been one of the RCP's larger and more active branches. William 'Mobile' Shaw was a local leader who recently passed and received public commendation from the party."
http://en.wikipedia.org/wiki/Revolutionary_Communist_Party%2C_USA
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"I maintain it was somewhat understandable, if not acceptable. So I call it a rebellion."--Maxine Waters