Officials said the blast was an accident which happened as troops were moving munitions at a base in Bidganeh, near the town of Shahriar, some 45 km west of Tehran.
The explosion shook homes and rattled windows for miles around, at a time of mounting tension with Israel over Iran's nuclear program.
The explosion started a fire at the base which raged for hours. Surrounding streets were closed and reporters were kept away from the scene.
Some media reported there had been two explosions and the head of Iran's Red Crescent organization said there was a risk of further blasts.
There were no reports linking the blast to any air strike or other attack. Tension has risen in recent weeks between Iran and Israel and the United States over nuclear-related issues.
Tehran denies Western accusations that were given some credence by a report from the UN nuclear agency this week, that its nuclear program has military ends.
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