The attack was carried out in or near a restaurant in the city centre, with witnesses saying that a number of people were injured.
No group has so far said it carried out the attack.
It comes as Somali government troops say they are advancing on the port of Kismayo - the key stronghold of the Islamist group al-Shabab.
The UN refugee agency has reported a sudden spike in the number of civilians fleeing Kismayo, from a few dozen to more than 1,000 a day.
Although al-Shabab has lost ground recently, it still controls large swathes of southern and central Somalia.
The BBC's Ibrahim Aden in Mogadishu says he saw six bodies at the Village restaurant, which is opposite the national theatre.
He says the restaurant is popular with civil servants and journalists, and two correspondents are reportedly among the dead.
Ali Mohammed Yassin, who was in the restaurant at the time, told the AFP news agency that he heard "a loud explosion".
He added that at least two people were taken to hospital with severe injuries.
Mohamed Sheikh, a security guard at the national theatre, also said that he saw bodies and wounded people, according to Reuters.