Israel came to a virtual standstill at sundown Friday as Jews began observing the start of the holiest day of the Jewish calendar, the 25 hours of fasting and contemplation known as Yom Kippur.

Though most Israelis are not religious, Israel all but shuts down for the duration of the fast day. There are no TV or radio broadcasts, businesses are shuttered and the streets are so devoid of cars that thousands of children take advantage of the day to ride their bicycles down highways.

Police began preparing during the early morning, deploying road blocks along main traffic arteries surrounding Jerusalem and along the northern and southern borders in efforts to prevent infiltrators from entering Israel.

Magen David Adom emergency medical staff were on standby ahead of the fast, as were the fire department and municipal centers.

Police also deployed in mixed Arab-Jewish neighborhoods in anticipation of possible clashes. In Acre, one of Israel's largest mixed cities, police was on especially high alert in light of violent clashes that erupted on Yom Kippur two years ago when an Arab resident drove through a Jewish neighborhood.

The high alert level will be maintained across Israel until Sukkot, which takes place next week.

Polls show that most Israeli Jews fast on Yom Kippur, also know as the Day of Atonement, and the Friday editions of the papers carried tips on how to prepare: have an egg for breakfast, drink 10 cups of water during the day and eat lots of carbohydrates.

The fast ends and normal life resumes at sundown on Saturday with a blast of the shofar, a traditional ram's horn, in synagogues across the country.

In Jewish tradition, Yom Kippur is the day on which God assesses the deeds of each person and decides their fate for the coming year. Jews must spend the day fasting, praying and repenting for past sins in hope of being forgiven.

Many will attend long services in synagogue. Rabbis from a group called Tzohar, which aims to make Judaism more accessible to secular Israelis, sends volunteers to organize prayer services in community centers and other locations to ensure that everyone has a place to pray.

"In Israeli society, which is mostly secular, Yom Kippur has remained sacred on some level," said Nachman Rosenberg, one of the group's leaders. "Many people can relate to ideas like calculating your actions and forgiveness."

Many Israeli Jews who do not observe Jewish law the rest of the year will not watch television, speak on the telephone or surf the Web on the fast day, even if they do not attend services.

Life continues, however, in the towns and neighborhoods that are home to the one-fifth of Israelis who are Muslims and Christians.

Because of concerns that Palestinian militants could carry out attacks, Israel's military closed crossings from the West Bank into Israel, barring the entry of Palestinians for the duration of the fast day.

Such closures have become routine on Jewish holidays, but West Bank tensions have also risen in recent weeks, parallel to the launch of a new round of Mideast peace talks. Hamas militants, who have threatened to derail the talks with violence, killed four Israeli settlers earlier this month, and Israeli troops killed a Hamas militant in a West Bank town early Friday.