Honoring the man who inspired his political career, President Obama today hailed Nelson Mandela as the "last great liberator of the 20th century" and urged the world to continue his life's work for justice and equality.

"To the people of South Africa - people of every race and walk of life - the world thanks you for sharing Nelson Mandela with us. His struggle was your struggle," the president told a crowd of thousands at a memorial service for the former South African leader in Johannesburg. "His triumph was your triumph. Your dignity and hope found expression in his life, and your freedom, your democracy is his cherished legacy."

Comparing him to the great leaders who came before him, Obama likened Mandela's legacy to that of Mahatma Gandhi, Martin Luther King, Jr., and Abraham Lincoln.

"In the arc of his life, we see a man who earned his place in history through struggle and shrewdness; persistence and faith. He tells us what's possible not just in the pages of dusty history books, but in our own lives as well. Mandela showed us the power of action; of taking risks on behalf of our ideals," Obama said.

"It took a man like Madiba to free not just the prisoner, but the jailor as well; to show that you must trust others so that they may trust you; to teach that reconciliation is not a matter of ignoring a cruel past, but a means of confronting it with inclusion, generosity and truth. He changed laws, but also hearts," he said.

Calling himself a beneficiary of Mandela's struggle for equality, Obama said his perseverance to end apartheid "stirred something in me."

"It woke me up to my responsibilities - to others, and to myself - and set me on an improbable journey that finds me here today. And while I will always fall short of Madiba's example…he makes me want to be a better man," he said.

"With honesty, regardless of our station or circumstance, we must ask: how well have I applied his lessons in my own life?" he asked.

Obama warned world leaders that "we cannot allow our progress to cloud the fact that our work is not done."

"Around the world today, we still see children suffering from hunger, and disease; run-down schools, and few prospects for the future. Around the world today, men and women are still imprisoned for their political beliefs; and are still persecuted for what they look like, and how they worship, and who they love. That is happening today," he said to cheers from the exuberant crowd, braving the rain to celebrate Mandela's life.

"There are too many people who happily embrace Madiba's legacy of racial reconciliation, but passionately resist even modest reforms that would challenge chronic poverty and growing inequality," Obama cautioned. "There are too many leaders who claim solidarity with Madiba's struggle for freedom, but do not tolerate dissent from their own people. And there are too many of us who stand on the sidelines, comfortable in complacency or cynicism when our voices must be heard."

President Obama was one of nearly 90 world leaders who traveled to South Africa to pay tribute to Mandela and one of six who delivered remarks at the memorial. Former presidents George W. Bush, Bill Clinton, and Jimmy Carter were also in attendance.