According to the provincial police, the explosion took place in Ganj area.
There were no casualties left to Afghan forces, said the police spokesman Ahad Walizada.
Arif Jalali, head of Herat regional hospital, said two people wounded in the explosion were taken to hospital.
There was no claim of responsibility for the attack.
The country, already gripped by decades-long war and violence, has seen a surge in bomb attacks, mostly claiming civilians.
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