The bomb – which injured more than 50 others – exploded near government headquarters.
In Baghdad, two car bombs went off at around the same time, killing at least nine people and wounding another 15.
Karbala is home to two of the most important shrines for Shia Muslims and has been often targeted by insurgents.
The latest violence comes as Iraq’s politicians try to form a new government – months after the country’s elections.
Source: BBC News