* Does not want to be obtrusive: "I don't need a photo op"
* S.Africa trip to be tribute to ill anti-apartheid leader
* Says he sees Africa as partner, not a "charity case" (Adds details about South African visit)
U.S. President Barack Obama paid tribute to anti-apartheid hero Nelson Mandela as he flew to South Africa on Friday but played down expectations of a meeting with the ailing black leader during an Africa tour promoting democracy and food security.
White House officials hope Obama's three-nation tour of Africa - his first substantial visit to the continent since taking office in 2009 - will compensate for what some view as years of neglect by America's first black president.
The health of Mandela, the 94-year-old former South African president clinging to life in a Pretoria hospital, dominated Obama's day even before he arrived in Johannesburg.
"I don't need a photo op," Obama told reporters aboard Air Force One after leaving Senegal. "The last thing I want to do is to be in any way obtrusive at a time when the family is concerned with Nelson Mandela's condition."
Mandela's ex-wife, Winnie Madikizela-Mandela, said his condition had improved in the past few days.
Nearly 1,000 trade unionists, Muslim activists and South African Communist Party members marched through the capital to the U.S. Embassy, where they burned an American flag and called Obama's foreign policy "arrogant and oppressive."
Muslim activists held prayers in a car park outside the embassy. Leader Imam Sayeed Mohammed told the group: "We hope that Mandela feels better and that Obama can learn from him."
MANDELA A "PERSONAL HERO"
Obama sees Mandela, also known as Madiba, as a hero. Whether they are able to meet or not, officials said his trip would serve largely as a tribute to the anti-apartheid leader.
Like Mandela, Obama has received the Nobel Peace Prize and both men were the first black presidents of their nations.
Air Force One departed Senegal's coastal capital, Dakar, just before 1100 GMT (0700 ET) and was due to arrive in South Africa around eight hours later. On Friday evening, Obama has no public events scheduled and could go to the hospital then.
"When we get there, we'll gauge the situation," Obama told reporters.
Obama was scheduled to visit Robben Island, where Mandela spent years in prison under South Africa's former white minority regime.
He told reporters his message in South Africa would draw from the lessons of Mandela's life.
"If we focus on what Africa as a continent can do together and what these countries can do when they're unified, as opposed to when they're divided by tribe or race or religion, then Africa's rise will continue," Obama said.
White House officials said Obama would hold a "town hall" on Saturday with youth leaders in Soweto, the Johannesburg township known for 1976 student protests against apartheid.
He will discuss a new exchange program for African students with U.S. colleges and universities. The event will include youth in Uganda, Nigeria and Kenya participating through video conference, and will be televised in those countries, White House officials said.
JAB AT CHINA
Obama's only previous visit to the African continent was a one-day stopover in Ghana at the beginning of his first term.
While acknowledging that Obama has not spent as much time in Africa as people hoped, the White House is eager to highlight what it has done, in part to end unflattering comparisons to accomplishments of predecessors George W. Bush and Bill Clinton.
"Given the budget constraints, for us to try to get the kind of money that President Bush was able to get out of the Republican House for massively scaled new foreign aid programs is very difficult," Obama said.
Obama and the Republican-controlled House of Representatives have fought bitterly over government spending. U.S. foreign aid is a perennial target for lawmakers who want more budget cuts.
Before departing Senegal, Obama met farmers and local entrepreneurs to discuss new technologies helping to raise agricultural output in West Africa, one of the world's most under-developed and drought-prone regions. The technical aid in the U.S. government's "Feed the Future" program leverages money from the private sector and aid groups to help small farmers.
Obama said he would announce an initiative to use the same strategies for the power sector, a model he said makes the most of the shrinking U.S. foreign aid budget.
"I think everything we do is designed to make sure that Africa is not viewed as a dependent, as a charity case, but is instead viewed as a partner," he said.
Obama acknowledged that China, Brazil, India and other countries have been increasingly active in Africa and said the United States risks being left behind. But he said the U.S. approach to development is preferred by African leaders.
"They recognize that China's primary interest is being able to obtain access for natural resources in Africa to feed the manufacturers in export-driven policies of the Chinese economy," Obama said.
"Oftentimes that leaves Africa as simply an exporter of raw goods" as opposed to creating long-term jobs, he said.