Martin Luther King Jr. gave his famous "I Have a Dream" speech at the culmination of the March on Washington on August 28, 1963. In it, he refers to acts of injustice against African Americans in three different places. Sometimes he speaks generally and sometimes more specifically.

First of all, he points out that one hundred years after Abraham Lincoln issued the Emancipation Proclamation, African Americans were still victims of injustice. As examples of the continuing lack of freedom of black Americans, King specifically mentions the problems of segregation, discrimination, poverty in the middle of prosperity, and banishment to the "corners of American society." He claims that the "unalienable rights of life, liberty, and the pursuit of happiness" have been denied to Black people.

Later in his speech, King exclaims that African Americans will never be satisfied until certain things are accomplished. By implication, the dissatisfaction that he feels is because these acts of injustice were still prevalent. The specific injustices he mentions include the prevalence of police brutality, the lack of access for Black people to motels and hotels, the confining of African Americans to ghettos, children being stripped of their dignity and selfhood when confronted "by signs stating, 'for whites only,'" the inability of African Americans to vote in Mississippi, and the apathy of Black voters in New York.

Finally, when King declares "I have a dream" and begins to delineate those future things that he sees in the dream, his implication is that these things do not yet exist. The lack of existence of these things implies further acts of injustice against African Americans. His dreams include equality for all, people of all races sitting down together in brotherhood, and people being judged by the "content of their character" instead of the "color of their skin."

In his "I Have a Dream" speech, Martin Luther King discusses the injustice of freeing black people from slavery but not, one hundred years later, granting them equality with white people: in the early 1960s, black people were still segregated in many states, were living, King said, in a "lonely island" of poverty amid general white prosperity, and were still treated as "exiles" in their own country.

King specifically asks that African Americans be freed from suffering police brutality, allowed access to all motels and hotels in the country (which at the time often would not take black customers), have the right to live where they wished, not isolated in slums and ghettos, and be allowed to vote in the state of Mississippi. He wants segregation and hatred toward black people merely for being black to end. He dreams of an integrated society, where all people are judged by who they are not, by the color of their skin.

While I agree with pohnpel397 that, in his speech, Martin Luther King Jr refers in general to injustices, a listener can draw some specific situations from his references.

When he refers to the Emancipation Declaration of 100 previous to his speech, and the continuing suffering of the African American in the "manacles of segregation", he is referring to each of the individual laws in counties and states around the nation that keep African Americans segregated from equitable access to wealth and resources. This he sees as injustice on a national scale.

The very fact that this speech was delivered at a March on Washington for Jobs and Freedom reinforces the idea that there was a systemic miscarriage of justice against African American citizens.

When King mentions police brutality, he is referring to all incidents of violence by law enforcement against African Americans and against people working for civil rights. The most famous of these was on 3 May 1963, just months before King's speech, when police used firehoses and attack dogs to repel a non-violent crowd of African American adults and children in Birmingham.

There is no doubt that those listening to King's speech were able to conjure up images of these individual cases of injustice, when he spoke in general terms. In fact, due to the alarming number of cases of injustice, the use of generalisations allowed King to communicate with the widest possible audience about his cause.

In this speech, King does not cite any specific acts of injustice towards any individual African Americans.  Instead, he speaks in general terms about the sorts of injustices suffered by all African Americans.  King talks about police brutality.  He talks about segregation in motels and hotels.  He talks about residential segregation and "white only" signs on drinking fountains.  He talks about the denial of the right to vote.  This is as specific as King gets in this speech.  He is talking about general kinds of injustice, not about specific incidents of injustice to individuals.