A car bomb exploded at a gate of police headquarters in the Afghan city of Kandahar on Thursday, a police official said.
There was no immediate word on casualties.
"The attackers have targeted the counter-narcotics wing in the Kandahar police," said Tadeen Khan, the southern city's chief of police.
After the blast, militant gunman opened fire from nearby positions and members of the security forces were battling them, Khan said.
No militant group claimed responsibility.
Police cordoned off the area as passersby fled.
US officials and Taliban militants have been discussing a peace deal since late last year but despite the talks, the level of violence has not fallen off.