By Kathryn Westcott
Public transport networks in major cities are increasingly the target for terror attacks.
The blasts on the Underground and on a bus in London come 15 months after the devastating train attacks in Madrid.
In March 2004, 10 bombs exploded on four trains in three stations in the busy morning rush hour, killing 191 people and leaving at least 1,800 injured. The attacks are believed to have been carried out by Islamic extremists.
London is the latest city to suffer attacks on its transport network
According to Dr Jonathan Eyal, director of studies at the Royal United Services Institute in London, there are obvious similarities between the attacks in London and Madrid.
"Like the attacks on the Spanish capital, the targets in London were key transportation nodes: underground stations which intersect with main railway stations, feeding through hundreds of thousands of passengers an hour during peak time," says Dr Eyal.
"And, like in Madrid, the purpose was to kill as many people as possible, by striking at different targets at more or less the same time."
The attacks in Spain, with the near simultaneous detonation of 10 devices, are said to rank among the most sophisticated rail attacks.
Investigators have discovered that the Madrid bombing devices were set off remotely using mobile telephones.
Three months after the Madrid bombings, an alleged Egyptian bomb expert was arrested in Milan in connection with a suspected plot to attack the Paris metro. Security experts say the plans were modelled on the Madrid bombing.
Experts discovered the Madrid devices were set off remotely using mobile telephones
Investigators believe that the alleged plan included the use of a program downloaded from the internet which would enable text messages to activate mobile phones simultaneously.
Mobile phones work underground in several of the largest Paris metro stations.
Transport systems are increasingly targeted because they are used by millions of people a day and are easily penetrated because they lack the security of, say, an airport. And, given the typical passenger density in a railway or subway station, substantial casualties can be inflicted with a relatively small device.
According to leading US experts in rail security, attacks against railways are more numerous and deadly than those on airports and planes.
Such attacks have increased in the past decade.
At a recent Rail Industry Safety Conference held in the US, experts noted that there had been more than 181 attacks on trains and related rail targets worldwide between 1998 and 2003, in such countries as Colombia, India, Spain, Pakistan, the UK, the US, and Venezuela.
Bombs were the most frequently used weapons in these attacks.
Last year, Moscow was rocked by two such blasts - one on a packed subway train that killed at least 39 people and injured more than 100, the other outside a central Moscow subway station which killed 12 people and injured more than 100. The Russian government blamed both attacks on Chechen rebels.
Paris, too, has suffered a series of bombing campaigns against its transport system. In the summer of 1995, the French capital suffered a number of attacks, beginning with the bombing of a packed commuter train in July 1995 that killed eight people and injured 100.
Although there were various claims of responsibility for the blasts, suspicions centred on Algerian Islamic extremists in response to the French government's policy towards the conflict in Algeria. Almost a decade earlier, Islamic extremists carried out a 10-month bombing campaign against targets in France, including a number against the public transport system.
In 1997, Islamic extremists plotted suicide bombings in New York's subways. Four years earlier, similar groups had planned to set off truck bombs in New York's tunnels and bridges.
The devastating attack in Tokyo in 1995 exposed subway systems to be uniquely vulnerable and raised the spectre that attackers of the future might resort to weapons of mass destruction.
Members of the Aum Shinrikyo religious sect killed 12 people and caused illness or injury in 5,000 after they released sarin nerve gas in the underground railway system.
One of the worst attacks against public transport systems was at Bologna station in 1980, when a huge bomb killed at least 75 people.
It was one of the deadliest attacks in Italian history and has become known in the country as the Strage di Bologna - the Bologna massacre.
It was widely believed the attack was carried out by right-wing extremists.