Police seized nearly 50 pounds of plastic explosives with detonation systems attached, along with 10 rifles and guns, six laptop computers and other evidence, police said.
Twelve people -- two Chechens, two Azerbaijanis and eight Turks -- were arrested in two raids, police said.
The raids -- which occurred in February but were only now reported -- occurred in the northwestern city of Tekirdag and Istanbul, police said.
All 12 people were believed to be members of al-Qaida terrorist cells, the Dogan News Agency said.
The U.S. Embassy issued a travel warning at the time but said police had provided no specific threat information about the targets.
It had no immediate comment Friday.
The U.S. Embassy was the target of a suicide bomb attack in February that killed a Turkish security guard and severely injured a local resident. But that attack was attributed to an extreme left-wing organization, not Islamic militants.
The arrested alleged attackers also planned to bomb an Istanbul synagogue and the private Rahmi M. Koc Museum, police said. They additionally intended to attack Turkish TV personality-actor Acun Ilicali and author Adnan Oktar, also known as Harun Yahya, an Islamic creationist who speaks against evolution, the news agency said.