The Rev Bernice A King recalled her father as a civil rights leader and great orator whose message of peaceful protest is still vital decades later.
"We decided to start this day remembering the apostle of non-violence," she said during a ceremony to award the Martin Luther King Jr Nonviolent Peace Prize held at the King Centre in Atlanta.
In Memphis, where King died, hundreds of people bundled in hats and coats gathered early for a march led by the same sanitation workers union whose low pay King had come to protest when he was shot.
The Memphis events are scheduled to feature King's contemporaries, including the Rev Jesse Jackson, the Rev Al Sharpton and US representative John Lewis, along with celebrities such as the rapper Common.
In the evening, the Atlanta events culminate with a bell-ringing and wreath-laying at King's crypt to mark the moment when he was gunned down on the balcony of the old Lorraine Motel on April 4 1968. He was 39.
Donald Trump issued a proclamation in honour of the anniversary, saying: "In remembrance of his profound and inspirational virtues, we look to do as Dr King did while this world was privileged enough to still have him."
The president has been the target of veiled criticism by some speakers at King commemorations in recent days as they complained of fraught race relations and other divisions.
The anniversary of King's death coincides with a resurgence of white supremacy, the continued shootings of unarmed black men and a parade of discouraging statistics on the lack of progress among black Americans on issues from housing to education to wealth.
But rather than despair, the resounding message repeated at the commemorations was one of resilience, resolve and a renewed commitment to King's legacy and unfinished work.
Wednesday's events followed a rousing celebration the night before of King's "I've Been To the Mountaintop" speech at Mason Temple Church of God in Christ, Memphis. He delivered this speech the night before he was assassinated.
Inside the church, Ms King called her older brother, Martin Luther King III, to join her in the pulpit, and she discussed the difficulty of publicly mourning their father - a man hated during his lifetime, but now beloved around the world.
"It's important to see two of the children who lost their daddy 50 years ago to an assassin's bullet," said Ms King, now 55. "But we kept going. Keep all of us in prayer as we continue the grieving process for a parent that we've had yet to bury."
A gospel singer led a rousing rendition of Lift Every Voice And Sing, and the gathering took on the air of a mass meeting.
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