Officials said a bomb-filled vehicle exploded outside the courthouse, part of a provisional government complex, while a trial of Taliban suspects was proceeding, the BBC reported.
Taliban insurgents claimed responsibility for the attack.
The vast majority of the estimated 70 people wounded were civilians, officials said.
Mohammad Yunus, Farah's deputy governor, told a private Afghan television station militants detonated a "large explosive-laden vehicle" that damaged both government and non-government buildings.
After the initial assault, militants positioned themselves in several buildings, Yunus said.
An Afghan security source told the BBC, "First there was a suicide attack, later on a group of attackers started fighting."