Police found and defused more than 600 kilograms (1,320 pounds) of explosives packed into a minibus that was parked near an Ankara market.
Bomb experts said the materials were similar to those seized in the past from Kurdish separatists. Media reports said police found sacks of bomb-making materials, speculating that it was ammonium nitrate or fertilizer, ingredients sometimes used by rebels to make explosives.
"A possible disaster has been prevented," Gov. Kemal Onal told reporters.
Sniffer dogs led officers to the blue minibus parked in a multilevel lot just meters (yards) from a food market, authorities said. Turkey, an ally of the United States in the fight against terrorism, had increased security on Tuesday, the sixth anniversary of the Sept. 11 attacks on the U.S.
The van's license plate had been stolen from another vehicle, and the owner had tipped off police about the theft, the state-run Anatolia news agency said.
A witness said an unidentified man had parked the minibus there at around 6 a.m. (0300GMT), saying he was bringing goods to sell at the market, Anatolia reported.
Police evacuated houses and cordoned off the area while explosives experts spent three hours defusing the bomb, Police Chief Ercument Yilmaz said.
Onal later said the bomb was larger than expected, private CNN-Turk television reported.
Yilmaz did not reveal the nature of the bomb. Both Kurdish rebels and Islamic militants have used ammonium nitrate-based bombs in attacks in the past.
Suicide bombers linked to al-Qaida hit Istanbul in 2003 with truck bombs containing ammonium nitrate-based 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.
Turkey, a predominantly Muslim but secular country is a key NATO ally. It supports U.S. operations in Afghanistan and Iraq through the Incirlik Air Base in the southern part of the country, one of the most important U.S. military bases in the region.
In May, a suicide bombing in one of Ankara's busiest markets killed six people and wounded dozens more.
The bombing was blamed on the separatist Kurdistan Workers' Party, or PKK, but the group denied involvement.
Kurdish rebels have been fighting for autonomy for Turkey's Kurds since 1984. Tens of thousands of people have been killed in the campaign against the government.