More than 100 people were wounded in the attacks, which came only minutes apart, at the Jamia mosque in the restive city of Zahedan, capital of southeastern Sistan-Baluchestan province bordering Afghanistan and Pakistan.
"Two explosions in front of the Zahedan Jamia mosque left more than 100 wounded and more than 20 martyred," Fariborz Rashedi, the head of the emergency services of Sistan-Baluchestan province, told state news agency IRNA.
Zahedan MP Hossein Ali Shahriari told Fars news agency the attacks were carried out by suicide bombers, with the first by a man dressed as a woman.
"The attacker, dressed in women's clothing, was trying to get into the mosque, but was prevented," Shahriari said.
"When people came to rescue those hit in that blast, another bomber blew himself up."
IRNA said the first attack was around 9:20 pm (1650 GMT).
Deputy Interior Minister Ali Abdollahi said it was carried out by a suicide bomber where worshippers were being frisked before entering the mosque.
"That attack resulted in the martyrdom of several, including some of the Revolutionary Guards," he said.
Abdollahi said the second attack took more casualties as people rushed to the site of the first bombing.
IRNA said the explosions were powerful because its reporter covering the blasts could see parts of bodies scattered in and around the mosque as medical workers collected them.
Window panes of nearby buildings were shattered from the impact of the explosions and two vehicles close to the mosque were also damaged, the agency said.
The head of Guards political bureau, Yadollah Javani, pointed the finger at United States, Israel and other Western countries as possible planners of Thursday's bombings.
"One cannot rule out the direct intervention of America, Zionists and other Western countries in the explosions at Jamia mosque of Zahedan," he told Fars.
In a joint statement Ayatollah Abbas Ali Sulemani, the local representative of Iran's supreme leader Ayatollah Ali Khamenei, and the provincial governor general Ali Mohammad Azad urged the residents to be "calm but alert against the enemies who wanted to divide the people," IRNA said.
Zahedan has been repeatedly hit by attacks blamed on Sunni rebel group Jundallah (Soldiers of God), which plays on feelings of resentment among ethnic Baluchis in the province.
The group's leader, Abdolmalek Rigi, was hanged on June 20 after he was captured in a dramatic operation by Iranian agents. Iranian officials maintain he had received support from US and British intelligence services for carrying out attacks against Iran.
Soon after his death, the group vowed to avenge his execution.
Jundallah says it has been fighting for nearly a decade to secure rights for Sunni Baluchis who form a significant proportion of the population in the province.
It claimed a suicide bombing last October that killed at least 42 people, including seven Revolutionary Guards commanders, in the town of Pisheen in Sistan-Baluchestan.
Jundallah also claimed a May 28, 2009 bombing against Shiite Amir al-Momenin mosque in Zahedan in which more than 20 people were killed and 50 wounded.
That attack came in the run-up to Iran's hotly disputed June presidential election which saw hardline incumbent Mahmoud Ahmadinejad win a second term.
Jundallah is also accused of a 2007 attack that killed 13 Guards.
Analysts estimate that the group was formed somewhere between 2000 and 2003 and has about 1,000 militants trained in small arms and explosives.
In recent years the Iranian authorities have resorted to increasingly tough measures against the outfit.
In July 2009 they hanged 13 of its members in a mass prison execution, terming them "enemies of God" after convicting them of a string of offences, including kidnapping foreigners.
The Revolutionary Guards were formed shortly after the 1979 revolution to defend the purity of the country's Islamic system. They have since become a major military, political and economic force in Iran.
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