A suicide car bomber has struck a police complex in the Somali capital Mogadishu, killing at least 30 people and wounding nearly 48 others.

Initial reports had put the toll at 12 dead and 14 injured, but more bodies were discovered in the rubble of the collapsed buildings on Sunday.

There were also more deaths as people succumbed to their injuries in hospital.

Most of the victims were labourers and port workers, said senior police officer General Abdullahi Hassan Barise.

Among the dead were at least five police officers and two young boys. Civilians made up most of the wounded.

The attack took place near the gate of Mogadishu's main port and is the latest in a series of militant attacks targeting the capital.

The bomber drove a minibus van packed with explosives into the police station and a security checkpoint, witnesses said.

The extremist militant group al-Shabaab took responsibility for the attack via statements on its radio station, Radio Andalus.

Al-Shabaab said the suicide bomber was targeting the police station, where a number of officers also lived.

The explosion severely damaged the building, according to police officer Ahmed Dhashane.

A busy tax facility was also affected by the blast.

The destruction also spread to nearby homes and vehicles. Thick plumes of black smoke could be seen above the blast site from throughout the city.

Gunfire could be heard from the police complex shortly after the explosion. Police said the situation was under control and security forces were using warning shorts to disperse people.

"The area has been surrounded by Somali security forces and African Union troops," said security forces spokesman Abdikamil Shukri Ma'alin.

Somalia is preparing to hold presidential elections on December 28.

AAP