London and the surrounding areas have been targeted by terrorists in the past, notably in the 1984 bombing of the Brighton Hotel, where five were killed in a failed assassination attempt on then-British Prime Minister Margaret Thatcher.
More recently, a nail bomb exploded in 1999 injuring some 45 people.
Many of the attacks on England's capital have been blamed on the IRA, or factions of the organisation.
In 1982, a bomb in Hyde Park killed two cavalryman and seven horses, while in Regents Park on the same day, seven members of the band Royal Green Jackets were killed when a bomb exploded under the stage.
A year later, a car bomb exploded at Harrods in London, killing six and injuring 90. The city was attacked again in 1992 when a bomb exploded at the London Bridge railway station, injuring 29.
On February 9, 1996, a lorry parked near South Quay station exploded, devastating a large area of London's Docklands.
Two people were killed.
Terrorism experts yesterday said leaving bombs on buses and trains was a soft target, aimed to disrupt one of the world's great financial centres.
Terrorism specialist Chris Bellamy said: "It has the co-ordination and the simultaneity of one group in particular, although it's too early to say. What we may be talking about here is not suicide bombers but just packages left on trains. It is the softest of targets. We've expected something like this for some time."
London, one of the world's most pre-eminent financial targets, and the capital of Britain, which had supported the US in the "war on terrorism", was a natural target, said Edwin Bakker at the Netherlands Institute of International Relations.
Anja Dalgaard-Nielsen, at the Danish Institute for International Studies, said: "It's shocking. It's always problematic to guess at such an early stage but co-ordinated bombs, that's the trademark of the networks that subscribe to al-Qaeda's ideology.
"The IRA has not carried out such attacks against civilians for ages, at least not without prior warning."
The London Underground, the world's oldest subway train network, is used by three million people a day.
Opened in 1863, the railway, familiarly called the Tube, today counts 274 stations spread along 400km of lines. The network runs deep under the British capital and is served by escalators that are among the longest in Europe.
Up to now, fires have created the biggest emergencies on the Underground.