Palestinian bomber blew himself up on a bus packed with ultra-Orthodox Jews, killing more than 20 people, and wounding about 100 in one of the deadliest bombings in the past three years of fighting. Palestinian Prime Minister Mahmoud Abbas condemned the bombing as a "terrible act." The blast went off shortly after 9 p.m. Tuesday, as the crowded tandem bus - which has two passenger sections - drove along a main thoroughfare in Jerusalem and another bus pulled in front of it, witnesses said. A second bus passing nearby when the explosion went off was also badly damaged, with windows blown out. Rescuers had to use blow torches to get out some of the wounded. "What is clear is that it was a very big bomb," said Jerusalem fire chief Amnon Amir. Paramedics treated wounded on the sidewalk, and body parts were strewn about. Hamas and Islamic Jihad both claimed responsibility for the blast. Police spokesman Gil Kleiman, Israel Army Radio and rescue service workers put the number of dead at 20. Israeli officials don't include the bomber in the death toll. Fourteen of the wounded were in critical condition. In Washington, the White House deplored the bombing and offered sympathies to the victims and their families. The explosion went off as Abbas was meeting with Islamic Jihad in the Gaza Strip to persuade them to halt attacks on Israelis. Abbas said he has asked Palestinian security forces to investigate. Palestinian Islamic groups declared a unilateral truce on June 29, but have said they would continue taking revenge for Israeli killings of their operatives. Last week, Islamic Jihad threatened attacks on Israelis to avenge the killing of a senior operative, Mohammed Sidr, in an Israeli arrest raid in the West Bank city of Hebron. **PHOTO CAPTION*** Israeli investigators work at the site of an explosion on a bus in downtown Jerusalem, Tuesday Aug. 19 2003. (AP Photo/Brennan Linsley)