The only person injuried was a rickshaw-puller who tried to open one of the sacks, causing it to explode.
The metal plates referred to an attack on the minority Ahmadiyas - a Muslim sect frequently targeted by radical groups within the majority Sunni Muslim community.
Ahmadiyas differ from mainstream Islam by not believing that Mohammad was the last prophet.
The messages also issued threats against Non-Governmental Organisations (NGOs) in Bangladesh.
"If Hazrat [Prophet] Mohammad is not declared the superman of the world by May 10, all non-governmental organisations will be blown up," the slogans on the metal sheets read in the Bengali language.
Police did not immediately confirm who were behind the blasts and no arrests have been made.
"We are investigating whether it is a new group [responsible for the bomb attacks]," said Nur Mohammed, national police chief.
Police said security across the country had been tightened after the bomb blasts.
In August 2005, three people were killed in a series of bomb blasts in towns and cities across Bangladesh organised by Jamaat-ul-Mujahideen, a banned group seeking the imposition of strict Islamic law.
Further bomb attacks by Jamaat-ul-Mujahideen and Jagrata Muslim Janata Bangladesh, another banned group, were staged through the rest of 2005, killing at least 30 people.
Six leaders from the two organisations were hanged in March after being convicted of the murder of two judges who died in the bomb attacks.
A state of emergency in Bangladesh has been in effect since January, when political violence forced the army-backed interim government to suspend a national election.