A local hospital was overwhelmed with the number of wounded being brought in hours after the attack, so they had to divert them to a second facility.
The United Nations secretary-general's office said that Ban Ki-moon has spoken with President Uhuru Kenyatta and expressed his concern. Meanwhile, Britain's Foreign Office urged British nationals to avoid the area, saying it is "urgently looking into" the incident and ready to provide consular assistance in case any British are involved.
Kenya suffered a spate of grenade attacks that killed more than 60 people from October 2011 to March 2013 after al-Shabab threatened attacks. Police attributed the attacks to sympathizers of al-Shabab in Kenya.
Authorities said they have thwarted other large-scale attacks targeting public spaces. Kenyan police said in September 2012 they disrupted a major terrorist attack in its final stages of planning, arresting two people with explosive devices and a cache of weapons and ammunition.
Anti-terror Police Unit boss Boniface Mwaniki said vests found were similar to those used in attacks that killed 76 people in Uganda who gathered to watch the soccer World Cup finals on TV in July 2010. Al-Shabab claimed responsibility for those bombings, saying the attack was in retaliation for Uganda's participation in the African Union's peacekeeping mission in Somalia.
In January 2012, Kenya said it had thwarted attempted attacks by al-Shabab over Christmas and the New Year.
Associated Press reporters Tom Odula in Nairobi, Kenya, Carley Petesch in Johannesburg and Cassandra Vinograd in London contributed to this report.
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