Officials blamed the blast on "anti-revolutionary" militants backed by foreign states.
Twelve people were killed and more than 70 injured on Wednesday when a bomb exploded among a crowd watching a military parade in northwestern Iran.
Officials blamed the blast, in the city of Mahabad in the predominantly Kurdish area near the borders of Iraq and Turkey, on "anti-revolutionary" militants backed by foreign states.
"This bomb was a time-bomb planted on a tree among the people and it went off at 10:20am ," the website of state-run television IRIB quoted a military official as saying.
There was no immediate claim of responsibility for the attack on the parade, which was part of "Sacred Defence" celebrations, an annual ceremony for the Iranian military.
"Counter-revolutionary groups, by inserting themselves among the people attending the armed forces parade, showed their heinous face," said Vahid Jalalzadeh, the provincial governor of Iran's West Azerbaijan province.
Television footage showed troops marching past a podium when a blast happened. Pictures of the aftermath showed blood on the ground, shoes and an abandoned pram. The government announced three days of mourning in the city.
The attack occurred as president Mahmoud Ahmadinejad was attending the United Nations General Assembly in New York amid a standoff between Tehran and major powers over Iran's nuclear programme, which the West suspects is aimed at making weapons.
"The blast was aimed at the president's political success in New York as well as Iran's defence power," Mehmanparast told the IRIB website. "It was a reaction of the Zionist regime (Israel) and its supporters (Washington)." Iran does not recognise Israel.
On August 4 a homemade explosive went off near Ahmadinejad's motorcade as he travelled to the western city of Hamadan. He was unharmed and officials said the blast was just a firecracker.
Several armed groups hostile to the government are active in Iran, including Kurdish separatists in the northwest, Baluch militants in the southeast and some Arabs in the southwest.
The Sunni Muslim Jundollah Baluch militant group, which Iran says has linked to al-Qaeda, is the most active. It claimed a double suicide attack that killed 28 people, including Revolutionary Guards, on July 15 in revenge for the execution of its leader.
In Mahabad, Jalalzadeh said Wednesday's explosion happened some 50 metres (160ft) from the podium and that two military officials' wives were among the dead.
No casualties were reported among the military personnel and political dignitaries attending the annual parade -- one of several held across the country to commemorate Iran's eight-year war with Iraq in the 1980s.
About one million people were killed in the war, a traumatic period in Iranian history but one which helped unify Iranians around the leadership of the then new Islamic Republic.
Iranian media have often reported clashes between government forces and Kurdish guerrillas said to be members of Party of Free Life of Kurdistan (PJAK), an offshoot of the Kurdistan Workers Party (PKK) which took up arms in 1984 for an ethnic homeland in southeast Turkey and northwest Iran.
Mahabad was the capital of a short-lived Soviet-backed "Republic of Kurdistan" in 1946, which was crushed within a few months. It was also the centre of a Kurdish uprising shortly after the 1979 Islamic Revolution.
Jalalzadeh echoed Iran's official stance, accusing Iran's arch foe the United states of supporting "counter revolutionary hypocrites and terrorist groups in the region".
"Today's action was one of the incidents which was (staged) by the conspiracy of foreign governments."
At the main national parade, held in Tehran before the bombing, the armed forces chief of staff blamed Washington for supporting the government's opponents inside Iran.
"(US secretary of state) Hillary Clinton has expressed concern and disappointment with (the US) inability to take military action against Iran and decided (instead) to create sedition inside Iran," Hassan Firouzabadi said.
Clinton recently voiced support for Iranians who oppose a regime that she said was "morphing into a military dictatorship".
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