During the trip, mobile phone users will be expected to use a local SMS short code in Ghana (1731) , Nigeria (32969) , South Africa (31958) and Kenya (5683).

The speech will be streamed live at http:www.whitehouse.gov/live, on Facebook and Twitter. The much-hyped trip is expected to raise the level of local content and access to social media as a communication tool.

"This visit will improve internet usage and raise the importance of social media in engaging the public," said Rita Mutuku, a media consultant in Nairobi.

This is Obama's second official visit to sub-Saharan Africa -- in 2006, he visited Kenya, Chad, Ethiopia, South Africa and Djibouti as a U.S. senator.