"We have issued orders for an investigation into this criminal act and for those who carried out this cruel crime to be detained so that justice can be done."
Jalal Talabani, Iraq's president, described the attack as a "cowardly terrorist act" by Sunnis fighters who had been displaced by the current security crackdown by Iraqi and US forces.
Talabani's office said: "They have committed a crime in a secure part of our country after they were besieged and kicked out of Anbar, Diyala and Samarra."
The governor belonged to the Supreme Islamic Iraqi Council (SIIC), formerly known as the Supreme Council for the Islamic Revolution in Iraq, and was a member of al-Maliki's coalition government.
Doctor Hamid Gaati, head of the health directorate, said: "The hospital in Diwaniya received the governor and police chief, and three other corpses from their security detail."
Brigadier-General Othman al-Farood, an army commander, said a driver and a bodyguard were among the other victims.
Jasim Azawi, presenter of Al Jazeera's Inside Iraq, said the attack was definitely a "targeted assassination".
"These are the top two most senior military and security officers in Diwaniya, this will be a major setback for the al-Maliki government," he said.
A senior security official in Diwaniya, which is about 130km south of Baghdad, told the AFP news agency that the convoy was hit by multiple explosions.
"More than 10 IEDs [improvised explosive devices] targeted a convoy of the governor and the chief of police on their way back from Aajaf to the centre of the city, killing the governor of Diwaniya and the chief of police," the official said.
Iraqi state television said the deputy governor had ordered an indefinite curfew from 7pm (1500 GMT) in Diwaniya after the killings in what the channel called an isolated area without security protection.
The province has seen frequent clashes between Shia groups and US and Iraqi troops. Violence between rival Shia factions has also been on the increase in the area.