The explosion rocked Jam-e-Mosque and more than 7 people lost their lives and at least 15 were injured.
Abdul Ahad Walizada, spokesman for the Herat police, said the bomb went off as people made their way to the mosque for prayers.
Najib Danish, the Ministry of Interior acting spokesman, confirmed to the Khaama Press explosives were planted near the entrance of Jame mosque in Herat city.
Bordering Iran, Herat is among the few relatively safer cities in Afghanistan.
That blast caused serious damage to the German embassy, and lighter damage to the Indian and other embassies nearby.
"We were not the only targets, the entire diplomatic community was the target of this attack", Ghani told foreign diplomats gathered for the peace conference in Kabul.
"The death toll coud rise", he said. The first blast, on last Wednesday, also took place in Kabul's diplomatic enclave and killed at least 150 people while 300 others were injured, Afghanistani President Ashraf Ghani informed North Atlantic Treaty Organisation global peace conference on Tuesday.