An explosion in a police compound in Turkey's southeastern city of Diyarbakir wounded several people and sent a large plume of grey smoke rising over surrounding buildings on Tuesday, but the cause of the blast was not immediately clear.

A police source said the explosion appeared to have taken place in a vehicle repair section of the police compound, causing part of the roof to collapse and wounding at least four people. The Dogan news agency said one was in a critical condition.

The blast comes ahead of a hotly contested referendum on Sunday on broadening President Tayyip Erdogan's powers, a constitutional change opposed by many in the country's predominantly Kurdish southeast.

The explosion was in the central, largely residential district of Baglar, where a car bombing by suspected Kurdistan Workers Party (PKK) militants wounded scores of people last November.

Diyarbakir is the largest city in Turkey's southeast, where the PKK has fought an insurgency against the state for more than three decades to press demands for Kurdish autonomy. Violence has flared since a ceasefire collapsed in July 2015.