The government said people had used hose pipes and jerry cans to collect the petrol, which had also spilt around the truck when the blaze erupted.
The tanker exploded in Tete province, a remote western region near the border with Malawi, killing 43 people on the spot, with more than 100 others badly burnt, including many children.
"For now, the total number of deaths is up to 80," Tete's deputy hospital director Veronica de Deus told the daily O Pais newspaper, adding that 35 people were still in a critical condition.
The truck was reported to be carrying fuel from Mozambique's port city of Beira to landlocked Malawi.
The government in Mozambique, one of the world's poorest countries, recently increased the price of fuel after the value of the local currency, the metical, fell sharply.