Iran's security service blamed the bombings - the deadliest in Iran in more than a decade - on supporters of ousted Iraqi leader Saddam Hussein.
State-run television quoted hospital officials as saying at least eight people were killed and 86 injured in four bomb explosions in Ahvaz, capital of the southwestern Khuzestan province bordering Iraq.
Hours later, two small bombs exploded in central Tehran, killing one person and wounding four. Police said one suspect was taken into custody.
A spokesman for the Supreme National Security Council, Iran's top security decision-making body, blamed groups affiliated to Saddam's former Baathist regime in Iraq. State TV quoted spokesman Ali Agha Mohammadi as saying the perpetrators of the Ahvaz bombings had infiltrated into Iran from Basra in southern Iraq.
Some Sunni leaders in Iraq have accused Shiite Iran of meddling in Iraqi affairs by backing Shiite Muslim clergy and politicians in a bid to sway Iraq's politics toward an Islamic establishment.
Iran denies the allegations, but some speculate that extremists loyal to Saddam could be trying to create insecurity in Iran ahead of the presidential polls.
"Whoever is responsible for this, the target of the blasts is to undermine Friday's presidential elections," said Interior Ministry spokesman Jahanbakhsh Khanjani.
Others pointed to a more local cause. Ahvaz was the site of recent violent protests over alleged plans to alter the proportion of Arabs and non-Arabs in the region.
Amir Hossein Motahar, director of security at the Interior Ministry, said one car bomb went off in front of the Ahvaz governor's office. He said two separate bombs blew up inside the toilets of the city's housing department and planning department. The fourth bomb, planted in a handbag on the street, exploded as experts tried to defuse it. The fourth site was near the home of the head of the provincial radio and television station.
Television footage showed heavily damaged buildings and blood on the ground. The force of the explosions also damaged many cars in the streets.
Gholamreza Shariati, deputy provincial governor for security affairs, said intelligence and security officials were investigating the explosions, but said the target was "Iran's territorial integrity ... on the verge of presidential elections."
Ahvaz also was the site of two days of violent demonstrations in April after reports circulated of an alleged plan to decrease the proportion of Arabs in the area. Officials at the time confirmed one death but opposition groups said more than 20 demonstrators had been killed. Some 250 were arrested.
The protests were sparked after copies of a letter allegedly signed by Vice President Mohammad Ali Abtahi circulated in the area. The letter ordered the relocation of non-Arabs to the Ahvaz to make them the majority population. Abtahi denied writing the letter.
Provincial spokesman Horri said security officials did not rule out the possibility of a connection between the blasts and recent protests.
Hadi Yunesi, a local journalist, said one of the buildings targeted was the provincial planning department, the authority the alleged Abtahi letter had been addressed to.
Yunesi said a list of names was circulated in the city recently, identifying people who cooperated with security officials in ending April protests, including radio and television officials. The anonymous letter circulated in Ahvaz had threatened to kill those named in the list.
"It's very likely that the explosions are connected to April's violent protests," Yunesi said.
Arabs make up about 3 percent of Iran's population; Persians account for 51 percent of the population of 69 million.
Bomb explosions have been rare in Iran since the end of the 1980-88 Iran-Iraq war.