Two bombers are also thought to have died in the attacks on Tuesday.
Quoting Israeli radio and medical sources, Aljazeera.net correspondent Ahmad Fayyd said 80 people were wounded. Of these five were in a critical condition and 12 in serious but stable condition.
The Palestinian group Hamas claimed responsibility for the attacks.
Reacting to the explosions, the Palestinian Authority condemned it and called for an immediate cessation of such attacks.
The explosions occurred on the two buses on the main intersection near the municipality offices of Beir al-Saba, the largest city in southern Israel.
The bombers targeted two buses
in a central part of Beir al-Saba
The city is 40km west of Gaza City.
The dead and the wounded were in the two buses, Aljazeera correspondent Walid al-Umari said.
Israel TV showed images of the two gutted buses, with flames shooting through the roof of one of the vehicles. At least two bodies, covered in white sheets, lay on the ground nearby.
Minutes later, police reported a third explosion, but rescue workers said it was a false alarm.
Interests not served
Palestinian Prime Minister Ahmad Quraya said: "The Palestinian government reiterates that our higher national interests are not served by such actions and they should be stopped immediately."
"The Palestinian government reiterates that our higher national interests are not served by such actions..."
Palestinian prime minister
He said such attacks give Israel a pretext to "assassinate" Palestinian fighters, step up military raids into Palestinian cities and speed up expansion of Jewish settlements in occupied territories.
Quraya's statement repeated a demand to Israel to accept an
"immediate and mutual ceasefire and negotiations on final-status issues", which includes Palestinians' quest for statehood.
"These explosions show that all the security precautions and the barrier have been unable to stop the attacks," Aljazeera correspondent al-Umari said.
Palestinian fighters have not carried out a major attack inside Israel since 14 March when 11 people were killed in the port of Ashdod.
Palestinian ambulance attacked
Earlier on Tuesday, Israeli troops opened fire on an ambulance at a military checkpoint in the southern Gaza Strip wounding a doctor and his driver, medics and a human rights group said.
Palestinian medics have
complained of taking Israeli fire
Palestinian medics said the two had sustained moderate injuries when troops opened fire on their ambulance which had been waiting for over three hours at the Abu Ghuli checkpoint, just outside the southern town of Khan Yunis.
A spokesman for Physicians for Human Rights (PHR) confirmed the information, saying the ambulance had been travelling northwards from Khan Yunis when it was seized at the checkpoint and held there for three hours without any justification.
"It took half an hour to evacuate the two men and when they arrived at hospital, the doctor had lost a lot of blood," PHR's Shabtai Guld said.
"We condemn this incident and ask the army to conduct an immediate investigation and to bring to justice anyone who fires on an ambulance," Guld said, adding that the group would file an official complaint.