A senior security official said the explosive devices had been planted in a bag by the roadside in the Azrati Baba area of the city.
A child and at least two women were among the dead and a World Food Programme vehicle was damaged in the attack on Thursday, but none of its passengers were hurt.
American and Romanian forces based in Kandahar cordoned off the busy commercial area, which is crowded with shops and restaurants.
Kandahar was the main stronghold of the Taliban before it was ousted in a US-led offensive in late 2001.
The explosion occurred as US Secretary of State Condoleezza Rice began talks with Afghan leaders in the capital, Kabul.
About 18,000 US-led troops are in Afghanistan fighting a Taliban-inspired insurgency in the south and east of