The attack came as security forces and protesters clashed in Kabul amid rising tensions over militant violence.
The assault began with a suicide car bomb outside the base just before the start of the morning’s rush hour.
Dozens of people were waiting to collect their monthly salaries ahead of the upcoming religious holiday.
A Taliban bomber drove into a crowd in Lashkar Gah, detonating a blast so forceful that some victims were blown into a nearby river, officials said.
A special police unit was dispatched after other armed attackers reportedly entered the site, authorities said.