Israel said it had foiled an "advanced" al-Qaeda plan to carry out a suicide bombing on the US embassy in Tel Aviv and bomb other targets.
Analysts said was the first time the global terror network's leadership had been directly involved in plotting an attack inside Israel.
The Shin Bet intelligence agency said it had arrested three Palestinians who plotted bombings, shootings, kidnappings and other attacks.
It said the Palestinian men, two from Jerusalem and one from the West Bank, were recruited by an operative in the Gaza Strip who worked for al-Qaeda leader Ayman al-Zawahiri.
The State Department said the US was not yet able to corroborate the Israeli claims.
While several groups inspired by al-Qaeda have carried out attacks against Israel before, this appeared to mark the first time an attack was directly planned by al-Qaeda leaders.
The Shin Bet said the Palestinians planned to attack a Jerusalem conference centre with firearms and then kill rescue workers with a truck bomb. Al-Qaeda also planned to send foreign militants to attack the US embassy in Tel Aviv on the same day, using explosives supplied by the Palestinians.
It said five men whose identity and nationality were not disclosed were to fly into Israel with fake Russian passports to attack the American embassy.
The Palestinian operatives also planned on several other attacks. One included shooting out the tyres of a bus and then gunning down passengers and ambulance workers.
The agency said it the plot was in "advanced planning stages" but gave no further information on how close the men got to carrying it out. It said the Palestinians from Jerusalem had used their Israeli resident cards to scope out and gather intelligence on targets. They were arrested in the past few weeks.
Several al-Qaeda-inspired groups have carried out rocket attacks from Gaza and Egypt's Sinai Peninsula, as well as shootings in the West Bank. Israeli intelligence calls these groups part of a "global jihad" movement.
Aviv Oreg, a former head of the Israeli military intelligence unit that tracks al-Qaeda, said the plot marked the first time it had been directly linked to an attempted attack in Israel.
"This is the first time that Ayman al-Zawahiri was directly involved," he said. "For them, it would have been a great achievement."
Al-Zawahiri's location is unknown, but he was last believed to be in Pakistan.
He is the subject of an intense manhunt and is not believed to personally go online or pick up the phone to discuss terror plots, experts say.