A ceremony has been held in Nairobi, the scene of an attack by al-Qaeda in 1998 which killed 213 people, mostly Kenyans.
Several hundred people attended a high-security, invitation only service, including a government minister and officials from the US embassy, which was blown up in the attack.
Wednesday has been declared a public holiday in both Liberia and Gambia, reports the French news agency, AFP.
Gambian President Yahya Jammeh, a devout Muslim, condemned those who use religion as a cover for acts of violence.
Two Gambians were killed in the attacks on the World Trade Center.
'Slave to revenge'
Kenya's Health Minister Sam Ongeri told those gathered at the Consolata Shrine parish church that Kenya was still committed to the US-led war against terror.
"I appreciate the outpouring of sympathy and support from the government and people of Kenya, given that Kenya has been subject to horrific attacks," said US ambassador Johnnie Carson.
But Roman Catholic nun Claudette La Verdiere implored God "to free them [the US] from the enslaving desire of revenge".
Kenyan victims of the 1998 bombing on Tuesday held a small protest, demanding compensation from the US Government - as it has given to the families of those killed in the 11 September attacks.
Mozambique's President Joaquim Chissano repeated his country's sympathies for the victims of the 11 September attacks.
But he urged the US to continue its war on terror "within the framework of the United Nations".
'Greatest crime'
Several South African newspapers echoed Nelson Mandela's call on the US not to take unilateral action against Iraq.
"Far from making the world safe from terror as he undertook, Bush's conduct has brought the world to the brink of a new Gulf War," says Business Day.
The Star has a front page photograph of Ground Zero.
But inside, Health Minister Manto Tshabalala-Msimang comments:
"I have been at many conferences and conventions where Americans were booed and humiliated by other nations. I think it is time America took notice of what other countries are thinking about."
Newspapers in Kenya, Nigeria, Senegal and Uganda have also used the anniversary of the 11 September attacks to urge restraint on George Bush.
"Terrorism will not be defeated through terrorism," says Kenya's Daily Nation.
Nigeria's Daily Trust says that any war against Iraq would rank as "one of the greatest crimes of the century".
"Only the hopelessly naive or wilfully ignorant could continue to seriously maintain that the military response of the US has been aimed at wiping out terrorism," it says.
Americans "have never asked themselves why they are so hated around the world," said Senegal's Walfadjri newspaper.