About 80 people were taken to hospital with injuries.
Al Jazeera's Alireza Ronaghi said the death toll was bound to rise as it was a very strong explosion.
"The timing of the explosion was interesting, as today was a national day of mourning in Iran for the death of the Prophet Muhammad's daughter," he said.
No one has claimed responsibility for the attack.
According to Irna, Ayatollah Abbas Ali Soleymani, Zahedan's prayer leader, told reporters that "one of the main persons involved [in the attack] had been arrested" and "he will soon be punished in front of the mosque".
Azad said that the bombers were aiming "to disturb the order in the province before the election considering the insecurity in the eastern neighbouring countries".
Presidential elections are due to take place in Iran on June 12.
The last major attack in the city was in February 2007 when suspected Sunni fighters killed 13 members of the Revolutionary Guards.
The authorities blamed the so-called Jundollah, or Soldiers of God, group for that attack.
Last month, Iran hanged three men for their involvement in a bombing inside a packed Shia mosque that killed 14 people in the southwest city of Shiraz in April 2008.
Sistan-Baluchestan province shares a border with Pakistan and is the scene of frequent clashes between Iranian police, drug dealers and armed groups.
The province lies on a major narcotics-smuggling route from Afghanistan and Pakistan.