A line of police behind riot shields hurled smoke canisters and fired pepper balls at as many as 200 protesters Tuesday night to enforce a citywide curfew, imposed after the worst outbreak of rioting in Baltimore since 1968.

Demonstrators threw bottles at police, and picked up the smoke canisters and hurled them back at officers. No immediate arrests or serious injuries were reported, and the crowd rapidly dispersed. It was down to just a few dozen people within minutes.

The clash came after a day of high tension but relative peace in Baltimore, as thousands of police officers and National Guardsmen poured in to prevent another round of rioting like the one that rocked the city on Monday.

It was the first time since the assassination of the Rev. Martin Luther King Jr. in 1968 that the National Guard was called out in Baltimore to prevent civil unrest.

The racially charged violence on Monday was set off by the case of Freddie Gray, a 25-year-old black man who died of a spinal-cord injury under mysterious circumstances while in police custody.

Maryland Gov. Larry Hogan said 2,000 Guardsmen and 1,000 law officers would be in place overnight.

"This combined force will not tolerate violence or looting," he warned.

In a measure of how tense things were on Tuesday, Baltimore was under a citywide 10 p.m.-to-5 a.m. emergency curfew. All public schools were closed. And the Baltimore Orioles canceled Tuesday night's game at Camden Yards and - in what may be a first in baseball's 145-year history - announced that Wednesday's game will be closed to the public.

The streets were largely calm all day and into the evening, with only a few scattered arrests.

About 15 minutes after the 10 p.m. curfew took effect, police moved against protesters who remained in the street in the city's Penn North section, near where a CVS pharmacy was looted.

Standing shoulder to shoulder, police in riot helmets began advancing toward the demonstrators in an effort to push them back. Some protesters lay in the street or hurled bottles toward the police. Then police used pepper balls and smoke.

Around the same time and in a different neighborhood, police tweeted that they were making arrests in South Baltimore after people started attacking officers with rocks and bricks. At least one officer was reported injured. (Associated Police)

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