Police and National Guard troops moved to disperse protesters in Baltimore on Tuesday night after a curfew took effect a day after the worst rioting in the United States in years.

As a week-long 10pm-5am curfew came into effect on 28 April, Baltimore police Captain Eric Kowalczyk said a "situation is unfolding" with an injured officer in the southern district of the city. He said no further information is available.

He said "there does not appear to be any connectivity" between the demonstrations and shootings Tuesday in the city.

Police in riot gear ordered a few hundred protesters to disperse, at an intersection in West Baltimore that was the scene of the worst rioting on Monday 27 April.

The crowd responded by throwing bottles and jeering at the police as the line of police began to advance.

Overhead, police helicopters also ordered the crowd to disperse.

Just ahead of the curfew, Mayor Stephanie Rawlings-Blake went to the intersection where protesters had gathered and pleaded with them to go home.

Kowalczyk said police definitely intend to enforce the curfew, and that they will do so with discretion, making exceptions for individuals in certain circumstances such as medical emergencies and commuting late to or from work.

Police used pepper balls at the intersection of Pennsylvania and North avenues just before 10:30 pm.

At the time, the crowd seemed to be mostly reporters, though perhaps several dozen protesters, some wearing bandanas over their faces, had also gathered.

At 10 p.m. when the curfew was set to go into effect, images on TV showed significant numbers of people still standing in the street and police lined up across from them, but not moving to make arrests immediately.

Just after the curfew began, dozens of young people still in the street to scattered throwing several bottles and rocks flew at the officers, hitting their riot shields.

Congressman Elijah Cummings spoke from a loudspeaker to urge the crowd to leave.

"There is nothing wrong with peaceful protest," Cummings said. "We all need to go home."

At 10:12, a helicopter overhead warned that the curfew was in place, and that everyone must leave.

"All news media, please clear the area," the police helicopter's message said, although reporters had been told by police earlier that they would not be arrested for staying out past the curfew. "You must go home or you are subject to arrest."

The Washington Post reported that "there's smoke in the intersection of Pennsylvania and North avenues. It's not clear whether police, protesters or both threw the projectiles that caused it."

Meanwhile, Hillary Clinton, who is standing for the Democratic Party nomination for President, said: "Baltimore is burning. It is heartbreaking. The tragic death of another young African-American man. The injuries to police officers. The burning of peoples' homes and small businesses. We have to restore order and security. But then we have to take a hard look as to what we need to do to reform our system."