Six bombs have exploded in Iran, killing at least 10 people, days before the presidential election.
The bombs in Ahwaz exploded over a two-hour period
Four blasts targeted public buildings in the south-western city of Ahwaz, killing at least eight people and wounding more than 70 others.
Hours later, a bomb exploded in the capital Tehran, killing two people. Three other bombs were defused.
Bombings have been rare in Iran since the war with Iraq ended in 1988. No group has claimed responsibility.
Ahwaz, which is close to the Iraq border, was the focus of unrest between Arabs and Persians in April, when several people were reportedly killed.
The bombings in Ahwaz took place over a two-hour period.
One of the bombs exploded outside the governor-general's headquarters.
Two went off near government offices and a fourth exploded near the home of a local state television executive.
The explosion in Tehran took place near the Imam Hussein square in the city centre. As well as the two who died, at least two people were wounded.
The interior ministry also confirmed that a bottle filled with explosives blew up in Vali Asr square in central Tehran, but there were no reports of casualties.
A spokesman for the Supreme National Security Council, Iran's top security decision-making body, blamed the attacks on separatist Arabs aided by members of the armed Iraq-based opposition group, the People's Mujahideen, and remnants of the Baath Party.
The spokesman, Agha Mohammadi, told the BBC he was sure the Americans were behind the attacks and also suggested that Britain might be involved - but he gave no evidence to support his claims.
The People's Mujahideen denied any involvement in the attacks.
"Whoever is responsible for this, the target of the blasts is to undermine Friday's presidential elections," said interior ministry spokesman Jahanbaksh Khanjani.
Iranians go to the polls on Friday to elect a successor to President Mohammad Khatami.
Opinion polls put former President Ali Akbar Hashemi Rafsanjani in the lead.
April's trouble in Ahwaz - the capital of oil-rich Khuzestan province - started after a letter circulated on the internet suggested that non-Arabs were being-relocated to the city to dilute its ethnic Arab population.
Crowds attacked government offices and banks, setting them on fire, and hundreds of people were arrested.
The official who was supposed to have written the letter said it was a forgery.