"The windows of my house shattered and three of my family members were injured," said Jassim Salih, a Beiji resident.
Falih Salim, another witness, said he rushed to the area after hearing the explosion and saw the bodies of two burned guards.
"The scene was so horrible that I ran away." he said.
In other incidents on Saturday, the US military said its soldiers had killed 12 suspected al-Qaeda fighters and detained 13 others in operations outside of Baghdad, the Iraqi capital.
Also on Saturday, Major-General Mark Hertling, the commander of US forces in north Iraq said that al-Qaeda fighters, driven out of other areas, were targeting police units in Diyala province.
But the spate of attacks over the last 24 hours did not reflect a wider surge in violence, he said.
"As far as an upsurge in attacks, we have not seen that," the Reuters news agency reported Hertling saying in an email.
Earlier this week, an al-Qaeda-linked group issued a threat vowing a wave of car bomb attacks and strikes against Iraqi security forces.