Officials said the 60 passengers on the bus had just arrived from St. Petersburg, Russia, to visit potential spots for tours and were heading for Eilat, a popular Red Sea resort.
The vehicle sped by in a no-passing zone, broke through a guard rail and rolled down the slope, said Rami Vazana, the driver of the bus that was passed.
"Dozens of wounded and dead were strewn along the slope. Most of them were thrown from the bus as it rolled," medic Gabi Baribo said.
Israel's rescue service said 24 people were declared dead at the scene, and Eilat's hospital said two more died later of injuries. Hospital officials said several more people were in critical condition.
The Russian ambassador to Israel, Pyotr Stegnii, told Russia's Vesti-TV that 30 people were killed.
It was the worst traffic accident toll in Israel's history. The previous high of 21 occurred in June 1985, when a train plowed into a bus stalled on the tracks, killing 18 children and three adults. In 1999, 16 people died in a bus accident in northern Israel that was similar to Tuesday's crash.
Irina Tyurina, a spokeswoman for the Russian Union of Tour Operators, told Vesti-TV that the visitors worked for five Russian tour agencies and were on a professional scouting trip to Israel.
Forty ambulances rushed to the scene and Israel's air force sent six helicopters to evacuate the seriously injured to hospitals across the country.
An Israeli military officer who was among the first to arrive said he and several others rescued six people trapped in the bus.
"They were saved because they were trapped in their seats. The rest were scrambled and mangled, spread out along the slope," said the officer, who identified himself only as Sawad because military regulations do not allow him to speak to the media without authorization.
Prime Minister Ehud Olmert "expressed shock at the horrific accident and the high number of people who were killed and wounded" and sent his condolences, according to a statement from his office. Olmert is in London on an official visit.
"Israel copes with traffic accidents on a daily basis and unfortunately not a week goes by without Israeli civilian motorists and pedestrians killed and injured," the statement said.
Before the bus accident, 406 people had been killed on Israel's roads this year, according to Or Yarok, a group that campaigns for traffic safety.
