Israeli police were on high alert Friday to avert possible clashes as Jews and Muslims prepare to observe their faith's major holidays of Yom Kippur and Eid al-Adha, which overlap this year for the first time in more than three decades.

Reinforced police contingents had been deployed across Israel as a precaution against sectarian violence, said police spokeswoman Luba Samri.

Concerns were particularly high in the four Israeli cities with significant Muslim minorities: Jerusalem, the Tel Aviv twin city of Jaffa -- Yafo in Hebrew -- and also in Haifa and Acre.

In a related development, the military said it had closed off the West Bank and the Gaza Strip as a general security measure, a common practice during major Jewish holidays.

Yom Kippur is Judaism's Day of Atonement, when devout Jews ask God to forgive them for their transgressions and refrain from eating and drinking, attending intense prayer services in synagogues.

Businesses and airports shut down and TV and radio stations go silent. Highways empty of cars, leaving many secular Israelis to ride their bicycles on the empty roads.

The holiday begins at sunset Friday and ends Saturday night.

Eid al-Adha, a three-day holiday that starts Saturday, is an occasion for family celebrations and outings. It commemorates the willingness of the prophet Ibrahim -- or Abraham as he is known in the Bible -- to sacrifice his son in accordance with God's will, though in the end God provides him a sheep to sacrifice instead.

On the start of Eid al-Adha, Muslims slaughter sheep, cattle and other livestock, and give part of the meat to the poor. Parents often buy new clothes for their children for the holiday.