And, as has been seen in England, the line between peaceful protest and store-burning riot can be very narrow.
"It's like shouting in a dark room. You don't always know who is there," Clem said. "Because we know people organize this way, we're listening."
While these groups may appear docile on the Internet, emotions often run high when they meet in person, and it only takes one small disagreement to turn a protest violent.
"You might get a lot of people who get upset about an issue very quickly and willing to do something with it in a moment's notice," Cotton said.
"It's on our radar," said Detective John White, a spokesman for the Denver Police Department.
The center started monitoring the sites in preparation for the 2008 Democratic National Convention in Denver, helping keep that event largely peaceful.
The system is not designed to prevent crowds from forming, but it does give law enforcement a chance to prepare for the worst.
...Eric Rosenberg, the president of Denver Flash Mob. Since the Denver group was founded in November 2010, Rosenberg has organized about a dozen flash mobs across the city—all of them peaceful.
"They keep getting bigger," Rosenberg said.
The largest local mob he ever assembled was last Saturday, when a group of about 150 people did a choreographed dance to support same-sex marriage.
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