John Lewis, the civil rights warrior who died Friday aged 80, excelled at what he liked to call "good trouble" -- standing up against racial injustice to forge a better United States. The African-American icon marched with Martin Luther King Jr., was nearly beaten to death by police, and later as a sitting congressman was arrested multiple times for protesting genocide or leading immigration reform sit-ins. "From a historical standpoint, there are few who are able to become giants," Martin Luther King III, the oldest son of the civil rights icon, told CNN. "John Lewis really became a giant through his examples that he set for all of us." Lewis was a sharecropper's son whose fights for justice helped define an era, and whose moral authority as an indomitable elder statesman left a permanent imprint in Congress. He was diagnosed with stage 4 pancreatic cancer in late 2019. White House press secretary Kayleigh McEnany tweeted on Saturday: "Rep. John Lewis was an icon of the civil rights movement, and he leaves an enduring legacy that will never be forgotten. We hold his family in our prayers, as we remember Rep. John Lewis' incredible contributions to our country." But Lewis had clashed with President Donald Trump on multiple occasions -- boycotting his inauguration and citing Russian interference in the 2016 election to question his legitimacy. Trump in turn said Lewis's Georgia district was "horrible" and the congressman was "all talk" and "no action." In a statement announcing he was ordering flags lowered to half staff at the White House and other public buildings and embassies for a day, Trump's only mention of Lewis was to say he was acting "as a mark of respect for the memory and longstanding public service of Representative John Lewis, of Georgia." He tweeted later: "Saddened to hear the news of civil rights hero John Lewis passing, Melania and I send our prayers to he and his family." - Risked 'life and blood' - Lewis was just 21 when he became a founding member of the Freedom Riders, who fought segregation of the US transportation system in the early 1960s, eventually becoming one of the nation's most powerful voices for justice and equality. He was the youngest leader of the 1963 March on Washington, in which King delivered his famous "I have a dream" speech. Two years later Lewis nearly died while leading hundreds of marchers across the Edmund Pettus Bridge in Selma, Alabama when state troopers, seeking to intimidate those demonstrating for voting rights for black Americans, attacked. Lewis suffered a fractured skull that day, which would become known as "Bloody Sunday." Fifty years later in 2015, he walked across the bridge arm in arm with Barack Obama, the nation's first black president, to mark the march's anniversary. Obama presented Lewis with the Medal of Freedom, among the nation's highest civilian honors, in 2011. "Not many of us get to live to see our own legacy play out in such a meaningful, remarkable way. John Lewis did," Obama tweeted early Saturday. "He loved this country so much that he risked his life and his blood so that it might live up to its promise," Obama added. Another civil rights giant also died Friday. Reverend CT Vivian staged anti-segregation sit-ins in the 1940s, was an early advisor to King and helped organize the Freedom Rides. He died early Friday at 95. - 'Conscience of Congress' - John Lewis was born in Troy, Alabama on February 21, 1940, the third of 10 children. His community was almost entirely black, and he quickly learned about the segregation that afflicted Alabama. Lewis, who organized sit-ins at segregated lunch counters and was arrested two dozen times for non-violent protests, was a founder and eventual chairman of the Student Nonviolent Coordinating Committee, where he wrote speeches against police brutality and campaigned to register black voters. He was elected to Congress in 1986 and quickly became a figure of moral authority. Tributes poured in from Democrats and Republicans alike. "Today, America mourns the loss of one of the greatest heroes of American history," House Speaker Nancy Pelosi said of the 17-term congressman. She described Lewis as "a titan of the civil rights movement." Republican Senator Mitt Romney called Lewis a man of "unwavering principle, unassailable character, penetrating purpose, and heartfelt compassion." In recent months, Lewis stepped away from congressional duties as he underwent cancer treatment. But he returned to Washington in early June, in the midst of fiery demonstrations following the police killing of George Floyd in Minneapolis, to walk in Black Lives Matter Plaza, the renamed intersection near the White House that was the site of protests against injustice. "The winds are blowing, the great change is going to come," Lewis said days earlier during a lawmakers' discussion on race.