At least three people were killed - apparently the man carrying the bomb who police said jumped off the bridge and the female shooters - and at least 10 people were injured, Egyptian officials and witnesses said.
After numerous brazen attacks during the 1990s, Egypt experienced a relative lull in such violence until October, when near simultaneous bomb blasts on two Sinai resorts killed 34 people.
Then, less than a month ago, a bomber targeted foreigners near a crowded Cairo bazaar.
An Interior Ministry statement identified the man killed in the explosion as Ihab Yusri Yasin, and said he jumped from the bridge during a pursuit, setting off the explosive he was carrying. He was being chased as a suspect in the 7 April bombing by the Khan al-Khalili bazaar, it said.
Security officials said police suspected that the two women involved in the shooting were relatives of Yasin or another suspect in the 7 April attack plot.
The security officials said they might have been related to Ashraf Said, a suspect in that bombing whom they said died in police custody a few days ago.
The bomber jumped off a bridge
and on to the victims
Health Minister Muhammad Awad Taj al-Din said he did not know if there were any connections between the two incidents.
He did not address the dead man's identity or circumstances of the blast, but said most of the injuries were superficial puncture wounds.
He identified the wounded as three Egyptians, an Israeli couple, both 55-years old; a Swedish man, 28; and a 26-year-old Italian woman.
The tour bus shooting was in old Cairo, an area rich with historic mosques and cemeteries.
At the site, a pistol and a black glove of the type worn by veiled women lay on the ground, amid blood and shattered glass.
Three Egyptians were wounded in that incident, one of them possibly a shooter. Witnesses said a police patrol in the area returned fire.
Police searched some of the
youths present at the attack
The bomb blast happened not far from a five-star hotel and 100m behind the Egyptian Museum. Remains of a body, covered with newspapers, were seen beneath the bridge a few minutes after the 3.15pm explosion was heard through downtown Cairo.
The blast happened on a road adjacent to a public bus station behind the museum.
Normally, the bus station is teeming with people heading home from work in the mid-afternoon, but the blast happened on a holiday weekend.
The hands and face of one of the injured - a man with reddish-blond hair who was lifted on to a stretcher - were covered with blood.
On a nearby curb, two Westerners checked their wounds; the young woman's left arm was bloodied and the man sitting next to her appeared to have sustained leg injuries. The extent of the other woman's injuries were not immediately clear.
A bomb attack earlier in April
killed three tourists
"The explosion was caused by a very primitive bomb full of nails. Most of the injuries were superficial caused by the destruction of the nails."
The Swedish man was in the worst condition, with injuries to his face.
Dr Hafidh Muhammad, deputy director of the French Qasr al-Aini Hospital where several of the wounded were being treated, said all would be released within days.
Initially, police said they believed a car had exploded, but no vehicle debris was seen in the area. A senior policeman on the scene, who would not give his name, said a bomb was thrown from the bridge above to the street below.
Some witnesses at the scene gave similar accounts. Later, the Interior Ministry said the man had jumped from the bridge with a bomb.
Two rings of tape cordoned off the area where the body lay. Investigators uncovered the body and knelt to inspect it while heavily armed police, including riot officers in helmets and carrying submachine guns, kept away the crowds who stood on benches and potted plants to get a view.
In a sign of the tension and uncertainty, police singled out a few youths to inspect the bags they were carrying.
In the second incident, which happened less than two hours later,Health Minister Muhammad Awad Tag al-Din said two women had opened fire on a tour bus and one of them was killed. He said three Egyptians were injured in that incident, including a woman.
The minister said it was not clear if the injured woman was the second shooter.
Tourists targeted again?
Police, who initially reported the second incident as an explosion, later said it was two veiled women who had opened fire on a tour bus. Senior police officials said both had been killed by security forces in the area. The differing accounts could not be reconciled.
No bus or bodies were seen in the area shortly afterwards, and there was no information about any damage to it or casualties aboard. There was shattered glass and blood on the ground.
Police inspect the bazaar in
The blast is the second bomb blast in the vicinity of major Cairo tourist attractions in less than a month. On 7 April, a bomber killed two French citizens, an American and himself when he detonated a homemade bomb near the Khan al-Khalili market.
During the 1990s, insurgents mounted several attacks on tourists in a bid to cripple tourism and bring down the government. The government has been anxious to limit the damage of recent attacks to Egypt's tourism industry, and has said the market blast was the act of only a few.
In October 2004, militants detonated bombs in the Sinai resorts of Taba and Ras Shitan, killing 34 people and wounding more than 100. One bomb destroyed a wing of the Taba Hilton Hotel.
Police said the mastermind was a Palestinian resident of Egypt who was angry with Israel. More than 10 Israeli tourists were among the dead.