Gov. Kemal Onal said a large amount of explosives were found stashed in a minibus, which had a fake license plate. The type of explosives found was not immediately clear, he said.
Bomb disposal experts were trying to eliminate the threat, but houses near the multistory garage were evacuated as a precaution.
"A possible disaster has been prevented," Onal told reporters at the site. "There is nothing to worry about now."
Police cordoned off the area and were also jamming mobile telephone signals in the area to block possible remote-controlled signals from triggering the bomb, private CNN-Turk television reported.
Suicide bombers linked to al-Qaida hit Istanbul in 2003 with truck bombs, killing 58 people in attacks that targeted two synagogues, the British Consulate and a British bank. In February, a court sentenced seven people to life in prison for the bombings.