The most likely attackers were Muslim militants, who have waged a campaign of violence for more than a year to overthrow the government and turn Egypt into an Islamic state.
Witnesses said the bus, carrying 49 Britons on a day trip from Cyprus, was driving through an underpass on the main road leading to the Pyramids of Giza when a man threw the bomb from a bridge above. It wrecked a Mercedes-Benz car driving in front of the coach and blew out the windows of the bus, which then crashed into the Mercedes.
'I was passing through the tunnel. In front of me was the Mercedes. Something was thrown from above and there was an explosion,' said Khaled Gad, the bus driver. 'I didn't stop, I drove quickly and came to the hospital.'
The Interior Ministry said an Egyptian man had been killed and nine Egyptians and five British tourists injured. Two of the Egyptians were in serious condition.
Doctors at a nearby hospital said four Britons had only slight wounds but one man had an operation to remove a nail embedded in his chest. The Egyptian tour guide was in intensive care with internal injuries.
John Cook, 45, who was treated for a cut in the head, said: 'We heard an explosion. The bus moved quickly . . . We came to the hospital. We got medical treatment and you can see that we are all right.'
A taxi driver, Nabil Fayez, said: 'I heard a big explosion just as the tourist bus was passing and I was behind him. After the Eexplosion, I saw grey smoke in the area. The tourist bus moved away fast . . . I stopped and saw blood over the hands of the twTHER write erroro passengers with me.'
More than 130 people have been killed in political violence in Egypt over the last 14 months. Muslim militants have attacked police, Christians and tourists, devastating the country's important tourist industry. Security forces have hit back with bloody raids on militant hideouts across the country and military courts have condemned 21 fundamentalists to death.
The massive security campaign has pushed the militants deeper underground and, until yesterday, appeared to have stopped attacks on foreigners. There had been no attempt against tourists since 11 April, when a bomb was found on a bus carrying a party of Germans at Cairo's medieval citadel.
But the militants have continued to hit other targets. They tried to assassinate the Information Minister, Safwat Sherif, in April and were blamed by the police for a bomb in central Cairo on 21 May that killed seven people, the bloodiest bombing in the recent wave of political violence.
Few of the militants' attacks on tourists have caused injuries. But they shot dead a British woman in a bus ambush in October and injured five Germans in a similar attack in November.