"Twenty-seven Guards personnel were killed in the explosion," a spokesman for the force, Commander Ramezan Sharif, told state television, adding that some of those injured were in critical condition.
An emergency official, Majid Khaled, quoted by the Mehr news agency, said 23 people were injured in the blast and taken to nearby hospitals.
The explosion shortly after 1 pm (0930 GMT) hit an ammunition depot at the base in Bid Ganeh, near the town of Malard on the western outskirts of Tehran, some 20 kilometres (12 miles) from the city centre, according to media reports.
It shattered the windows of residential neighbourhoods in the western suburbs of Tehran, witnesses told AFP. It was heard in the city centre.
"Initial investigations show the blast occurred as ammunition was being moved," Sharif said earlier.
The deputy head of the national security commission, Esmaeel Kosari, said parliament would open a probe into the blast, the ISNA news agency reported.
Hossein Garousi, a lawmaker from the area, ruled out the blast being the result of "an act of sabotage or in any way political." He told parliament's website the blast had destroyed "a large part of the ammunition depot."
Helicopters and ambulances were dispatched to the area, ISNA quoted an official at Tehran's medical emergency centre, Hassan Abbasi, as saying.
Earlier, a Mehr news agency reporter said two hours after the blast, a fire was still raging, and that there were huge traffic jams on roads leading to the base.
In October 2010, a blast at a Guards ammunition storage in the western city of Khorramabad killed 18 of its members and left dozens injured.
