Baltimore: At least 2,000 people marched through downtown Baltimore on Saturday to protest the unexplained death of a black man in police custody, and authorities said some demonstrators threw objects at officers and broke windows.

In the biggest protest since 25-year-old Freddie Gray died on Sunday, two separate clusters of protesters chanting ‘shut it down’ began marching. The groups merged into a single wave headed toward City Hall for a peaceful rally, the latest US demonstration protesting the treatment of African Americans by law enforcement.

After the rally, some groups of protesters fanned out across the city and disturbances were reported. Baltimore police said on Twitter that some protesters threw objects at officers and broke windows.

Local television showed footage of a protester throwing a crowd-control barrier toward officers.

Protesters also jumped on some police cars, breaking their windshields. At various times, protesters faced off against officers in front of Camden Yards, home of the Orioles baseball team whose evening game against the Boston Red Sox began as scheduled.

At least two arrests were made during the protest, Baltimore police spokesman Captain Eric Kowalczyk told local television station WBAL. The on-camera interview quickly ended after a group of demonstrators began yelling at Kowalczyk.

Gray was arrested on 12 April and died a week later, the latest US fatality of a black man under questionable circumstances during police encounters. The deaths have triggered an outcry over the use of force by law enforcement against African-Americans.

Last year, weeks of protests followed the shooting death of unarmed black teen Michael Brown in Ferguson, Missouri, and the death of Eric Garner, a black man in New York City who was placed in a chokehold during an attempted arrest.

On Friday, Baltimore's police commissioner conceded that police had failed to provide Gray with timely medical attention for a spinal injury he had suffered sometime after he was apprehended and put inside a transport van. Police have not explained how he sustained the injury.

During the march, some demonstrators confronted about 50 police officers in front of the baseball stadium. The protesters pushed against a wall of barricades and waved signs at officers, who stood silently in two lines. Some demonstrators kicked and dented police cars parked nearby.

‘The revolution is here! I'm going to kill you! All of you - guilty!’ one demonstrator yelled as he leaned over a barricade.

Traffic in the largely African-American city was blocked by masses of people gathered in the street.

Six Baltimore police officers have been suspended in the Gray case, and an internal police investigation is under way.

‘We are all united in our demand to indict the six police officers and convict,’ said Sharon Black, spokeswoman for People's Power Assembly, one of the rally organisers.

On Friday, Baltimore Police Commissioner Anthony Batts said the officers repeatedly failed to give Gray medical assistance and disregarded department regulations by failing to buckle the man into seat restraints in the van.

Gene Ryan, president of the Baltimore police union, criticized Batts' comments. Ryan, president of the Baltimore Fraternal Order of Police, said in a statement the commissioner's comments ‘appear to be politically driven’.

Police have said Gray fled when officers approached him in a high-crime area, but he was caught a short time later and placed in the van. He was carrying a switchblade knife, police said.

When the van arrived at the police station, an ambulance was called and Gray was taken to a hospital. He died a week later.

Batts said on Friday that investigators were still trying to determine what happened inside the police van. Police said their investigation would be completed by 1 May, a day before protesters plan another rally in Baltimore.

The department will turn over its findings to state prosecutors and an independent review will follow.