SAN FRANCISCO (Reuters) - The California evangelical broadcaster predicting Judgment Day will come on Saturday says he expects to stay close to a TV or radio to monitor the unfolding apocalypse.

Harold Camping, 89, previously made a failed prediction that Jesus Christ would return to Earth in 1994.

But the head of the Christian radio network Family Stations Inc says he is sure an earthquake will shake the Earth on May 21, sweeping true believers to heaven and leaving others behind to be engulfed in the world's destruction over a few months.

"We know without any shadow of a doubt it is going to happen," Camping told Reuters.

His Family Radio has 66 U.S. stations and broadcasts in more than 30 languages through international affiliates.

His supporters have posted about 2,200 billboards around the United States about the coming apocalypse, and dozens of followers have driven across the country to spread the news.

Camping, a civil engineer who once ran his own construction business, plans to spend May 21 with his wife in Alameda, in northern California, and watch the doomsday unfold.

"I'll probably try to be very near a TV or a radio or something," he said. "I'll be interested in what's happening on the other side of the world as this begins."

Like his last prediction, Camping's doomsday date is based on his reading of the Bible and a timeline dating back to ancient events including the Biblical flood survived by Noah.

Camping's pronouncement of a specific date for the apocalypse puts him outside the Christian mainstream.

Even Jerry Jenkins, co-author with Tim LaHaye of the "Left Behind" series of apocalyptic novels that have sold millions of copies worldwide, has a problem with the prediction.

"As a believer, I'm already a kook compared to most people, so for someone to choose a date and get everyone excited about a certain time, my problem is it makes us look worse," said Jenkins, 61.

A number of atheists are reacting to Camping's pronouncement, but probably not in a manner that would receive his blessing.

In Tacoma, Washington, atheists have organized a party for Saturday night at an arcade, under the banner "countdown to backpedaling," on the assumption that Camping and Family Radio will change their story if Judgment Day does not come.

At least 100 people are expected at the party, said Sam Mulvey, 33, an organizer of the event and the producer of a weekly atheist radio show in Tacoma.

"If the world still exists the next day, Family Radio is going to have to say something and most of the time they backpedal, and that's what we're counting down to," he said.

Mulvey said he has heard from atheists in other parts of the country who are planning barbecues for May 21.

Stephen O'Leary, an expert in religious communication at the University of Southern California, said the idea of rapture espoused by Camping and more mainstream Christians first appeared in Christian teaching in the 19th century.

"It is very appealing to people," said Barbara Rossing, professor of the New Testament at the Lutheran School of Theology at Chicago who describes a huge apocalyptic "prophecy industry" including video games, board games and books.

Tom Evans, a spokesman for Camping, said at least several tens of thousands of people listen to Family Radio's message.

One of those is Allison Warden, 29, of Raleigh, North Carolina, who most recently worked as a payroll clerk for an Ohio company and now runs the end-times website

"My boss does not agree with this but has been very understanding and supportive," she said. "He thinks next week I'll be back to work like normal."

(Additional reporting by Bob Mezan: Writing by Alex Dobuzinskis: Editing by Greg McCune)