A number of attacks this year, including one in September on a base where United States special forces train Somali commandos, show the group maintains a strong intelligence network and can mount deadly and sometimes sophisticated operations, analysts say.
Two Turkish citizens were among the dead, Turkey's Foreign Minister Mevlüt Çavuşoğlu confirmed shortly after the attack.
Police officer Mohamed Hussein said the blast targeted a tax collection centre during morning rush hours, Al Jazeera reported. Al-Shabaab, which is tied to al-Qaida, has carried out many similar attacks in the past.
The explosion occurred in a busy area prone to heavy traffic due to a security checkpoint and a taxation office.
Meanwhile, an ambulance service official Abdikadir Abdirahman Haji Aden told AFP that the blast killed at least 76 people, leaving 70 injured.
While the attack has not been claimed, experts said the auto bombing bore all the hallmarks of past attacks by al Shabaab militants allied to al Qaeda.
Russian Federation just tested its own internet: Here's what this means
MOSCOW-Russia has successfully tested Runet , a country-wide alternative to the global internet , its government has announced. Russia's project - nicknamed Runet - presumably follows suit and lets the government filter content through its own censors.
Earlier, Mukhtar said the blast also injured at least 60 others.
The militia group has regularly attacked government institutions around the troubled horn of Africa nation.
In February, the group claimed responsibility for a vehicle bombing at a crowded shopping mall that left at least 10 dead.
After the explosion, 55-year-old Sabdow Ali, who lives nearby, said he left his house and counted at least 13 people dead.
"Dozens of injured people were screaming for help, but the police immediately opened fire and I rushed back to my house", a witness told Reuters news agency.
Witnesses described a horrific scene of bloodied corpses strewn in and around the check point, and the injured shrieking for rescuers to pull them from beneath the scores of vehicles turned into rubble by the blast.
The police were not reachable for comment on casualty numbers. It was also behind three auto bombings last November that killed at least 52 people with about 100 more injured. The group had previously relied on military-grade explosives captured during assaults on an African Union peacekeeping force.