While there was no immediate claim of responsibility, attention immediately focused on the separatist group Eta, which resumed full-scale attacks in August after calling off a ceasefire.
A police officer told the Associated Press news agency that he did not know if there was a warning call before the blast - Eta often phones in warnings - or who the bodyguard had been assigned to protect.
Many public figures in the Basque region use bodyguards to protect against Eta attacks.
Spanish National Radio and other Spanish media said the guard was assigned to a Socialist member of the city council of the Basque village of Galdakao.
Speaking from Madrid, Benjamin Jones, a journalist, told Al Jazeera: "Eta usually does not claim its attacks for several days or perhaps weeks afterwards, but certainly the authorities are pointing their fingers at the armed Basque separatist movement."
He said: "Ever since the end of the ceasefire declared by Eta in June, it has attempted seven such attacks, [but] only one succeeded.
"Today the interior minister said he was upping security alert all around the country because of the approaching national holidays."