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 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 Dakar, Obama used his personal reflections of Mandela to help push democratic values at a time when sub-Saharan Africa has emerged as one of the fastest-developing regions in the world, and other countries, including China, have begun ramping up their own investments on the continent.

The president acknowledged the United States has been neglectful, but he pledged to support countries such as Senegal, which Obama held up as an example to neighboring countries for its pursuit of a free society whose government is held accountable.

In a meeting with Senegal President Macky Sall, Obama praised him for pursuing a case against former Chad dictator Hissene Habre, whose administration is accused of killing thousands, for crimes against humanity. And the president pledged to support Senegal's trial of Habre with resources, including a $1 million grant to the special court, administration officials said.

But the challenges that remain were apparent during a joint news conference when Obama hailed the U.S. Supreme Court's ruling this week that a law denying federal benefits to married same-sex couples is unconstitutional. Sall, asked by a reporter whether he would pursue changes to Senegal's law that criminalizes homosexuality, said he did not think his country was ready for such a change.

"These issues are societal," he said. "We should not have one standard model that's applicable to all nations."