Simultaneous bombings bearing the hallmarks of international terrorists ripped through crowds watching the World Cup final in Uganda's capital, killing 64 people.
One American was killed and several were wounded.
The deadliest attack occurred at a rugby club in Kampala last night as people watched the game between Spain and Holland on a large-screen TV outdoors.
The second blast took place at an Ethiopian restaurant, where at least three Americans were wounded.
One American was killed in the blasts, said Joann Lockard, a spokeswoman for the US embassy in Kampala.
Kampala's police chief said he believed Somalia's most feared militant group, al-Shabab, could be responsible for the attack. Al-Shabab is known to have links with al-Qa'ida, and counts militant veterans from the Iraq, Afghanistan and Pakistan conflicts among its ranks.
Body parts found at the rugby club suggested a suicide bomber may have been to blame, a reporter at the scene said.
At least three Americans - part of a church group from Pennsylvania - were injured at the restaurant. One was Kris Sledge, 18, of Selinsgrove, Pennsylvania.
"I remember blacking out, hearing people screaming and running," Mr Sledge said from the hospital, where his right leg was bandaged and he had burns on his face.
"I love the place here but I'm wondering why this happened and who did this. At this point we're just glad to be alive."
At the scenes of both blasts, blood and flesh littered the floor among overturned chairs.
Police chief Kale Kaihura originally said at least 30 people had been killed, though the toll could be higher.
Later, a senior police officer at the scene said that 64 people had been killed - 49 from the rugby club and 15 at the Ethiopian restaurant.
Mr Kaihura said he suspected al-Shabab, that country's most hardline militant group. Its fighters, including two recruited from the Somali communities in the US, have carried out multiple suicide bombings in Somalia.
If so it would be the first time the group has carried out attacks outside of Somalia.
Simultaneous attacks are also one of al-Qa'ida's hallmarks.
In Mogadishu, Somalia's capital, Sheik Yusuf Sheik Issa, an al-Shabab commander, said early today that he was happy with the attacks in Uganda but refused to confirm or deny that al-Shabab was responsible.
"Uganda is one of our enemies. Whatever makes them cry, makes us happy. May Allah's anger be upon those who are against us," he said.
During weekly Friday prayers in Somalia two days before the double bombing, another al-Shabab commander, Sheik Muktar Robow, called for militants to attack sites in Uganda and Burundi - two nations that contribute troops to the African Union peacekeeping force in Mogadishu.
In addition to its troops in Mogadishu, Uganda also hosts Somali soldiers trained in US and European-backed programmes.
White House spokesman Tommy Vietor said today the US was prepared to provide any necessary assistance to the Ugandan government.
"The president is deeply saddened by the loss of life resulting from these deplorable and cowardly attacks, and sends his condolences to the people of Uganda and the loved ones of those who have been killed or injured," Mr Vietor said.
Kenya's foreign minister Moses Wetangula said last week that enough veteran militants from the Iraq, Afghanistan and Pakistan conflicts had relocated to Somalia to spark worry inside the international community.
Foreign fighters have flocked to Somalia because the country's government controls only a few square miles of the Mogadishu, leaving most of the rest of the country as lawless territory where insurgents can train and plan attacks unimpeded.
US secretary of state Hillary Clinton said the US would work with the Ugandan government "to bring the perpetrators of this crime to justice".