Sistan-Baluchestan province, bordering Pakistan, faces serious security problems including bombings and frequent clashes between police and drug dealers and bandits.
"Border agents learned that subversive groups wished to enter our territory," said Ahmad Reza-Radan, Iran's deputy police chief, according to the Mehr news agency.
"The agents were successful in identifying and seizing a large shipment of explosives which had entered our country."
The haul included 20 bombs, Radan said. He did not say when exactly the explosives were found. One person had been arrested in connection but others had escaped to Pakistan.
"We request that the Pakistani government act on its obligations in controlling border regions, because its border has become a way for bandit traffic," he was quoted as saying.
Radan did not identify the arrested person or the suspected groups involved.
In the last major attack in Zahedan, the provincial capital of Sistan-Baluchestan, a suicide bomber killed dozens at a Shiite Muslim mosque in 2010.
That attack was claimed by militant Sunni group Jundollah (God's Soldiers), which accuses Tehran of discriminating against Sunnis. Since 2003 it has claimed a number of other attacks and kidnappings in Iran.
Tehran has linked Jundollah to the Sunni Islamist al Qaeda network and accused the United States of backing Jundollah to create instability in the country, a charge Washington denies.
Iran rejects allegations by Western rights groups that it discriminates against ethnic and religious minorities.