Gen. Garad Nor Abdulle, a senior police official said the members of the Qatari delegation who were being escorted in the interior minister's convoy were unharmed and safely reached their hotel.
Abdulle said the interior minister was not in the convoy.
Mohamed Abdi, an officer at the scene of the blast, said four civilians and a soldier died immediately. Another two people died in hospital and 18 were being treated of wounds from the blast, said Dr. Duniya Mohamed Ali at the Medina hospital.
The Qatari delegates are involved in development projects in Mogadishu, Somali president Hassan Sheikh Mohamud said.
Mohamud blamed al-Qaida-linked Somali militant group al-Shabab for the attack.
He said "suspects" have been arrested.
After the explosion soldiers fired in the air to disperse crowds that had gathered at the blast site at the busy KM4 junction.
Separately, four Somali soldiers were wounded Sunday when a roadside bomb struck a government vehicle in Deynile district, in Mogadishu's northwest, said Ali Jimale, a captain with the Somali police.
The Somali government reopened key roads in the Somali capital, Mogadishu, four days ago which had been closed for security reasons. The roads were closed after the government received intelligence that militants were planning attacks, officials said.
KM4 is among the busiest roads in Mogadishu, largely used by government officials and African Union forces. It connects the presidential compound and other government offices to the airport.
The car bombing falls into a pattern of attacks blamed on the Islamic extremist group al-Shabab, which has been pushed out of much of the areas it occupied in South and Central Somalia by African Union troops.
Condemning Sunday's attack, the U.N. representative to Somalia, Augustine P. Mahiga said cowardly and senseless acts of violence will not undermine the remarkable progress Somalia has made in the past months.
"Attacks against civilians are never justifiable. I call on all parties to renounce violence and contribute positively to peace and stability," he said.
The British government condemned the attack through its Minister for Africa Mark Simmonds.
He said incidents such as these demonstrate the importance of the Federal Government of Somalia and international partners working together to combat violent extremism in Somalia.
Next week's Somalia Conference in London, co-hosted by the British and the Federal Government of Somalia will provide international support to help build Somali capacity to increase peace and stability, said Simmonds.
Al-Shabab once controlled almost all of Mogadishu. African Union and Somali forces pushed the radical rebels out of the city in 2011, but the fighters have continued to carry out bomb attacks.
Al-Shabab claimed responsibility for an attack on Somali's Supreme Court last month that killed 35, including nine attackers.
Somalia's prime minister said that several experienced foreign fighters took part in attack on the Supreme Court, the most serious Islamic extremist attack on Mogadishu in years, while other officials indicated the explosive devices were more advanced than normal, a possible indication of greater involvement by al-Qaida.The attack included six suicide bombings and two car bombs.
Al-Shabab boasts several hundred foreign fighters, including some from the Middle East with experience in the Iraq and Afghanistan conflicts. Al-Shabab also recruits fighters from Somali communities in the United States and Europe.
In March, an explosives-filled car targeting a truck of government officials hit a civilian car and exploded, setting a mini-bus on fire and killing at least seven.
Somalia has not had a functioning government since 1991 when warlords overthrew longtime dictator Siad Barre and turned on each other, plunging the impoverished nation into chaos.
President Mohamud was elected by parliament in October at the end of the eight-year U.N.-backed transitional government.
The U.N.-backed political process that resulted in Mohamud's election was condemned by Islamist militants who said it was manipulated by the West. But Mohamud has the support of the international community, which wants him to succeed and bring stability to the troubled Horn of Africa nation.
Associated Press Writer Cassandra Vinograd contributed to this report from London.