It is hard to make out from the pictures, but it does look like the Secret Service pulled guns on Tea Party Protesters in Bristol (Virginia). That is also what the eye witness reports are saying;
"During the motorcade when the president was arriving, there were several vehicles following the limo that contained the secret service. All of the vehicles had all the windows rolled down, and back hatch open on the SUVs with the men holding their, I assume assault rifes, machine guns, drawn on everyone lining the streets. Needless to say it took my breath away at the sight of them, and made my friends and I dizzy with fear. I have seen the secret service before, but never like this. While they were intimidating, I never felt in danger. The guns were not drawn when the motorcade was leaving the event. But I turned on a local talk radio program as we were leaving and all the calls were about witnessing the guns being pointed at them and nothing else until the end of the program."
I know that it is standard operation for the president to be protected by a lot of people with some impressive firepower. I can not recall another incident where the security forces had the weapons drawn and pointed at protesters doing nothing more than waving signs as the president passed by.
Here is the pictures (same source):
"Destiny sent this picture of the caravan driving through Bristol:
Here's a close up of the secret service vehicle with the guns drawn on the protesters: