Israel Radio said the closure order applied to Jews and Christians but not to Muslims, who revere it as the third holiest site in their faith.
Israel TV said the decision to close the site known to Jews as the Temple Mount came after the Shin Beth security service received concrete information about a planned attack.
The closure order came before a planned demonstration by Israelis opposed to Prime Minister Ariel Sharon's Gaza withdrawal plan.
An Israeli rabbi says he expects thousands of Jews to take part in the Sunday demonstration.
Rabbi David Elboim says he is confident at least 10,000 Jews would heed his calls to gather at the site when the Jewish month of Nissan (April) begins. Police have banned the demonstration.
Violence at the site could spark reactions all over the Muslim world.
Israeli police have warned that Jewish ultra-nationalists hope to divert forces from Gaza to Jerusalem with a huge demonstration in the summer, scuttling the pullout.