Protest marches sprang up in cities across the United States on Tuesday, as a huge security operation stifled clashes in Ferguson, the town at the center of the country’s latest racially-charged riots.

Protest marches sprang up in cities across the United States on Tuesday, as a huge security operation stifled clashes in Ferguson, the town at the center of the country's latest racially-charged riots.
  
Violent unrest erupted in the St Louis suburb for a second night, after Monday's decision by a grand jury not to prosecute a white police officer for shooting dead an unarmed black teenager.
  
Missouri Governor Jay Nixon said the National Guard force in the Ferguson area had been tripled to more than 2,000 troops to back up officers' beleaguered local police force.
  
A St Louis police patrol car was burnt by protesters and the force declared the demonstration an "illegal gathering," warning marchers and journalists alike that they faced arrest.
  
Meanwhile, armed Missouri National Guard troopers sealed off West Florissant, the road running through Ferguson that was the scene of the worst looting and arson on Monday night after the verdict was announced.
  
At the Ferguson police station riot police dispersed around 100 protesters chanting and waving placards, including one that read: "We will not be silenced."
  
The crowd fell back towards Ferguson city hall, where a patrol car was set on fire and riot officers fired tear gas and deployed imposing armored personnel carriers to regain control.