I was walking past Freedom Plaza on Pennsylvania Avenue in Washington on Tuesday afternoon and saw a group of people were staging a die-in. The two dozen young men and women, of all races, were protesting the Monday night grand jury decision in Ferguson, Missouri, not to indict Darren Wilson, a white police officer who fatally shot black teenager Michael Brown in August.

On Tuesday evening, angry protesters in Washington also burned a US flag outside the American Portrait Gallery near Chinatown. Such were just two of the many protests and some riots that have taken place in many US cities in the last two days. Protests have actually never stopped in Ferguson in the 110 days since 18-year-old Brown was shot six times on Aug 9.

The deployment of thousands of National Guard members, the use of tear gas and the hundreds of arrests made by the police also show how tense the relationship is between protesters and law enforcement.

The debates in the United Stated have heavily focused on the loopholes in the grand jury decision, and how truthful Darren Wilson was in telling his story on ABC News on Tuesday. But the outburst of anger across the US reflects only one thing - people's deep frustration with the nation's racial inequality.

While such inequality is masked on the surface by Barack Obama being elected and reelected as the first African-American president, it often quickly reaches boiling point when triggered by incidents such as the ones regarding Brown or Trayvon Martin, a 17-year-old black teenager shot and killed in Florida in 2012.

Obama has tried to highlight the progress of racial relations in his speeches, yet his frustration was apparent when he said in July last year that, "Trayvon Martin could have been me 35 years ago". He expressed similar frustration on Monday night when he said that "the frustrations that we've seen are not just about a particular incident. They have deep roots in many communities of color who have a sense that our laws are not always being enforced uniformly or fairly".