"It is one of the worst attacks we have sustained [in the last 20 months] from the standpoint of casualties," Shlomo Aharonisky, the national police chief, told reporters.
Officials believe the level of damage suggests a powerful new type of car bomb is being used. Witnesses say debris and human remains were scattered for hundreds of yards down the highway.
Israeli Intelligence Says 'Mega-Attack' Planned
The use of a moving car bomb against a bus is widely believed to be a new form of suicide attack by Palestinian militants, who have earlier tended to use explosive belts strapped onto the waists of suicide bombers. In recent months, a few female suicide bombers have carried explosives in backpacks, presumably due to the weight and bulk of the belts.
Israeli military officials said Palestinian militants were trying to carry out another "mega-attack," like the March 27 bombing of a Passover Seder in the Israeli coastal town of Netanya.
Israeli army spokesman Brig. Gen. Ron Kitrey told reporters today that Maj. Gen. Aharon Zeevi, the Israeli military chief of intelligence, had told a parliamentary committee that militants had attempted to use cyanide gas in the Passover attack. However, Israeli experts believe the gas was not released to due a technical difficulty.
The bombing at the Park Hotel in Netanya killed 29 people and triggered Operation Defensive Shield, the Israeli military offensive in the West Bank earlier this year.
Meanwhile, White House spokesman Ari Fleischer condemned today's attack. "These terrorists are the worst enemies, not only of the people of Israel who seek peace, but also of the Palestinian people and their hopes for a better life," said Fleischer.
Today's attack underlined the importance of reforming the Palestinian Authority's security apparatus, Fleischer added.
CIA Director George Tenet is currently in the region to discuss the issue with Palestinian and Israeli officials. On Tuesday, Tenet met with Arafat to press the Palestinian leader to carry out reforms in the Palestinian Authority's bloated security system.
President Bush is expected to meet with Sharon in Washington next week. Amid reports that a Middle East peace conference was being planned in Turkey next month, Bush will also meet with Egyptian President Hosni Mubarak later this week.
ABCNEWS' Gillian Findlay in Jerusalem contributed to this report.