DAKAR, Senegal -- It was a brief but symbolically powerful moment.
President Barack Obama stepped alone into the frame of the Door of No Return on Senegal's Goree Island on Thursday afternoon, peering out at the Atlantic Ocean from the same vantage point that thousands of African slaves once did on their way to North America.
The United States' first African-American president was then joined by his wife, first lady Michelle Obama. Their daughters, Malia and Sasha, also took a turn.
For Obama, the tour of the former slave house was one in a series of emotionally weighty visits that the president intends to accentuate his bid to spread U.S. values and strengthen his administration's ties with three budding African democracies during a week-long trip.
"This is a testament to when we're not vigilant in defense of human rights what can happen," he told reporters at the scene.
Today, the president will continue his push in South Africa, where he will spend three days, highlighted by a tour this weekend of the Robben Island prison that housed anti-apartheid icon Nelson Mandela for two decades as a political prisoner.
White House officials have said Obama will defer to Mandela's family's wishes regarding a possible visit with the 94-year-old, who continues to battle a serious lung infection.
During a news conference in Dakar, Obama called Mandela a "hero" whose writings in defiance of South Africa's apartheid movement inspired Obama to a life of political activism when he was a 19-year-old Occidental College freshman three decades ago.
"I've had the privilege of meeting Madiba and speaking to him," Obama said, using Mandela's tribal name and referring to their 2005 meeting when Obama was a senator. "I think he's a hero for the world. And if and when he passes from this place, one thing I think we'll all know is that his legacy is one that will linger on throughout the ages."
In a statement on Thursday, South African President Jacob Zuma said Mandela's health had improved from critical to stable, bringing some relief to South Africans whose concerns grew on Wednesday as Zuma abruptly canceled an official visit to Mozambique to visit Mandela and confer with his doctors.