Khalil Asir, a police spokesman in the northeast province of Takhar, said explosives were placed in a motorcycle near the rally, which was to have been addressed by the candidate, Nazifa Yousufi Bek.
"So far we received a report of 12 dead and 32 injured, the victims include security officials and civilians," said Asir.
No militant group immediately claimed of responsibility for the attack.
Scores of people had gathered to listen to the candidate, but police said she was not at the rally at the time of the blast, Reuters reported.
Bek is one of 417 women contesting seats across the country, more than ever before, despite deadly suicide attacks on election rallies and offices apparently aimed at forcing voters to boycott the vote scheduled for Oct. 20.
Candidates, regardless of gender, are braving violence and opposition from social conservatives in a campaign seen as a test of the war-torn nation's democratic institutions.
Five candidates, all men, have been killed in separate attacks so far. Another two have been abducted, and four others have been wounded by hardline Islamist militants, election officials said.
Takhar, a province bordering Tajikistan has long been a hotbed of the Taliban insurgency. The Taliban has directed Afghans to boycott the upcoming polls.