U.S. National Guard troops were deployed Monday after unrest in a Missouri town where a white police officer shot dead an unarmed black teenager, as President Barack Obama called for calm

U.S. National Guard troops were deployed Monday after unrest in a Missouri town where a white police officer shot dead an unarmed black teenager, as President Barack Obama called for calm.

A clearly uncomfortable Obama said there was no excuse for local police to employ "excessive force" and urged the state to make only "limited" use of the guards, while still condemning violent protest.

By late afternoon, about 200 National Guard soldiers arrived at command headquarters, with snipers posted on rooftops in the surrounding area. The troops are operating under Missouri Highway Patrol supervision.

The reinforcements allowed State Governor Jay Nixon to lift an overnight curfew, but tempers were still running high amid ongoing controversy over the death of 18-year-old Michael Brown.

"They're supposed to protect the American citizens, but they're fighting a war with unarmed citizens," said Ron Henry, who wore a t-shirt with the phrase "stop killing us."

Amid the trouble, Getty Images photographer Scott Olson was arrested for unknown reasons, the agency said.

As night fell, several hundred people took part in fresh protests after police with plastic handcuffs moved crowds of demonstrators into designated zones to clear a main street in the town that had been the scene of violent protests.

Protesters were not allowed stop and gather, and were directed to keep moving.

Attorney General Holder, who briefed Obama earlier, is due to travel to Ferguson on Wednesday to meet with FBI agents and Justice Department officials conducting a federal civil rights probe into the shooting.