Speaking at the airport in Cameroon's capital, Yaounde, the Pope called on Christians to speak up in the face of violence, poverty, hunger, corruption and abuse of power.
While in Africa, the pontiff is expected to talk to young people about the Aids epidemic and explain to them why the Catholic Church recommends sexual abstinence as the best way to prevent the spread of the disease.
He gave a similar message to African bishops who visited the Vatican in 2005, when he told them that abstinence and fidelity, not condoms, were the means to tackle the epidemic.
The BBC's Caroline Duffield, in Cameroon, says people in Yaounde have been energetically sweeping and cleaning everywhere in preparation for Pope Benedict's visit.
The Pope also met an imam from Cameroon's Muslim minority
The Pope will stay until Friday in Yaounde, where he will meet bishops from all over Africa who will be taking part in a meeting at the Vatican later this year to discuss the Church's role in Africa.
In Angola, which is still recovering from 27 years of civil war, Pope Benedict will meet diplomats posted in Luanda and is expected to urge the international community not to abandon Africa.
The pontiff is also due to hold private talks with political leaders in the two countries, both of which have been accused of corruption and squandering revenues from natural resources.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.