Racial discrimination against black minorities is prevalent among Ferguson's police force, according to a damning new report.

"Ferguson's approach to law enforcement both reflects and reinforces racial bias, including stereotyping. The harms of Ferguson's police and court practices are borne disproportionately by African Americans, and there is evidence that this is due in part to intentional discrimination on the basis of race," the report by the US Department of Justice said.

"We have found substantial evidence of racial bias among police and court staff in Ferguson."

The report cites emails circulated by police supervisors and court staff that stereotype racial minorities as criminals. One email joked about an abortion by an African-American woman being a means of crime control, another said Obama wouldn't last long as President because "what black man holds a steady job for four years".

An email from October 2011 showed a photo of a bare-chested group of women dancing, apparently in Africa. The caption read 'Michelle Obama's High School Reunion".

The official investigation into police in Ferguson was launched after riots broke out in the Missouri suburb last year. Protestors took to the streets following the fatal-shooting of 18-year-old African-American student Michael Brown in August, which prompted allegations of racism among the city's police force. Almost the entire force are white, 50 of 54 officers, although two-thirds of the population are black.

The officer who shot Brown, Darren Wilson, was not indicted for the crime.

According to the report, 85 per cent of all stops and 93 per cent of all arrests between 2012 and 2014 in Ferguson were of African-Americans. The police department "routinely makes arrests without probable cause," and uses excessive force, it adds, offering numerous examples.

"The court's practices also impose unnecessary harm, overwhelmingly on African-American individuals, and run counter to public safety," the report concludes.

James Knowles III, the mayor of Ferguson, told a press conference on Wednesday that racist behaviour would not be tolerated by the police force or any other department in the city. One official who sent an offensive email had already been fired, and two others suspended pending an inquiry, he added.

Shane Claiborne, a Christian activist who has campaigned for racial equality and encouraged peaceful protest during the Ferguson riots, said that he was unsurprised by the report's findings. He said on Twitter that the "contagion of racism...has infected the police dept & courts".

Amnesty International USA executive director Steven W. Hawkins said in a statement: "Michael Brown's death touched off a long-overdue and much-needed conversation about race and policing that must continue. The US still has a long way to go before it has truly accountable policing. This country's long history of racial profiling and other police abuses is only matched by its equally long history of inadequate accountability for those responsible."

Hawkins called on the Obama administration to initiate comprehensive criminal justice reform across the nation.

"Ferguson is one of thousands of cities in this country with entrenched tension between law enforcement and the communities it is tasked to protect," he said.

"The Justice Department can and should move forward with a national review of police tactics and practices, especially those concerning deadly force. Not only that, it must begin to require all police agencies to collect and publish the statistics on deaths by law enforcement. We still don't know exactly how many people are killed by police officers every year."

The release of the report sparked another protest in Ferguson. A small number of demonstrators, thought to be around 50, gathered outside the city's police station on Wednesday evening, leading to four arrests. Critics have already condemned the police's actions.