In Nigeria, President Goodluck Jonathan has ordered all schools and government offices in the capital Abuja to close for three days during an international conference next week.
There have been two bombings in Abuja in the past fortnight. An explosion late on Thursday killed 19 people, and 75 were killed at a bus depot on 14 April.
The Boko Haram group is blamed. The same group is believed to be behind the kidnapping of more than 200 teenage girls from their school in Borno state on 14 April also.
The United States has warned of a plan to attack one of two Sheraton hotels near Lagos, the main commercial centre.
In a statement on its website, the State Department said those behind the plot were "groups with associated with terrorism", but gave no further details.
The World Economic Forum for Africa is due to meet in Lagos between 7-9 May.
Premier Li Keqiang of China and the presidents of Rwanda, Senegal and Kenya will be among those at the forum.
The government says 5000 police and soldiers will be deployed.
Boko Haram has never attacked Lagos, although its leader Abubakar Shekau has threatened to do so.