The Prince of Wales and Duchess of Cornwall have visited monuments to Abraham Lincoln and Martin Luther King Jr as part of their trip to the US.
They were joined by figures from the American civil rights movement, Jesse Jackson and Congressman John Lewis.
Later, speaking at an environmental conference, the prince called on governments and businesses to end the dumping of plastics into the oceans.
The couple will meet President Barack Obama at the White House on Thursday.
The couple - who arrived in Washington DC for their three-day visit on Tuesday - were given a guided tour of the Lincoln Memorial sculpture, pausing at the spot where Dr King gave his famous "I have a dream" speech in 1963.
At Dr King's nearby memorial, they met Rev Jackson and Congressman Lewis, who helped organise the famous civil rights march in Selma, Alabama, in 1965 - dramatised in the recent movie Selma.
The prince and duchess were also given a tour of Mount Vernon - home of America's first president, George Washington.
The duchess also paid a visit to Washington's Shakespeare Theatre Company.
Prince Charles also visited the National Archives, where he viewed the United States' Charters of Freedom and the 1297 version of the Magna Carta.
He later told leading delegates to an environmental conference on marine waste of his concerns about the "increasing quantity of plastic waste" in oceans.
He said he had been "haunted" by images of seabirds found dead after mistakenly eating plastic, and called for better recycling and disposal of plastics.
The solution to problems caused by a "throw-away society" was to move towards a circular economy, where "materials are recovered, recycled and reused instead of created, used and then thrown away", he said.
Nicholas Witchell, BBC royal correspondent in Washington DC
Ask most Americans who they think of when they contemplate the British Royal Family and the chances are that Prince Charles would not be among the top three, and possibly, if we include Prince George, not even in the top five.
That's been something of a problem for him over the past decade or so. He's very often been squeezed out of the picture by his mother, his sons and his daughter-in-law.
Yet this is the man who will be Britain's next king and who, even now, is taking on more of his mother's role, most especially on overseas visits.
So a visit to Washington, to the UK's most important international ally, by Britain's king-in-waiting, has a particular resonance.
At the Lincoln memorial, on a cold spring morning, he and his wife Camilla received a warm welcome from groups of American schoolchildren on their half-term break.
A good many of these American youngsters knew who they were: they may have struggled with the precise details of his title ("Prince of England?") and her name (Camilla isn't as easy on either the memory or the tongue as Diana), but most of them appeared to realise that this was the guy who was one day going to be a king.
As for Charles, he's probably more at ease with himself and his immediate surroundings than he has been for decades.
Partly that's because he appears genuinely to enjoy visits such as this, accompanied as he is by Camilla, and with a programme that fulfils both the needs of the Foreign Office in terms of bolstering Britain's links with opinion formers and power brokers, and which also - importantly for Charles - gives him a chance to speak out on issues which matter to him, principally the environment and its protection.
King-in-waiting he may be, an appointment with President Obama at the White House he may have, but don't think for a moment that Charles is going to curb his commitment to issues which, as he puts it himself, he's been "banging on about" for decades.
They matter to him more than ever, and Washington gives him a platform to pursue them.
The US visit will see also the royals join in commemorating several important milestones.
These include the 150th anniversaries of the end of the American Civil War and the death of President Lincoln, and the 800th anniversary of Magna Carta.
Prince Charles is also expected to receive an award for exceptional leadership in conservation from the International Conservation Caucus Foundation.
The royal couple will also visit the home city of boxer Muhammad Ali, who was born in Louisville, Kentucky.
They will tour the city, which has a cultural centre dedicated to the life and achievements of the three-time world heavyweight boxing champion.