Officials have put the number of fatalities at 13 but medics say the toll could rise even higher.
They said the explosion occurred when a motorbike – laden with explosives – went off near the gathering of supporters of a female election candidate in the northeastern province of Takhar.
Among the victims were both civilians and security forces.
Khalil Aser, the provincial police spokesman, said the attack took place before the candidate, Nazefa Yusoufi Beg, arrived at the rally.
The attack was the latest in a series of explosions targeting election campaign rallies. No group has yet claimed responsibility for the attack but the Taliban has already vowed to disrupt the long-delayed parliamentary vote scheduled for October 20.
The Taliban said in a statement on Monday that it had called on its followers to target security forces on the election day.
The militant group is largely believed to be trying to block the vote which it describes as “bogus” and a “malicious American conspiracy.”
The polls were originally set to be held in 2015 following presidential elections the year before but were repeatedly pushed back due to security fears and logistical problems.
Preparations for the ballot have been in turmoil for months and there has been widespread speculation about whether the vote would go ahead.
An Interior Ministry official, however, said earlier that the government’s security preparations would prevent the Taliban from following through on its threat.