A string of bombings targeting London’s transportation system also left 700 injured, Paddick said.
Paddick, who is deputy assistant commissioner of the city's Metropolitan Police, said authorities are not certain who was responsible for the three explosions in the subway system and one blast on a double-decker bus in central London.
"Four devices, we believe, were involved in today's incidents. The police service received no warning about these attacks, and the police service has received no claims of responsibility from any group in connection with these attacks," Paddick said.
The British government says the attacks are by terrorists, but that it is still too early to speculate about which group bears responsibility. The blasts came as leaders of the world's eight leading industrialized countries – the G-8 – met in Scotland for a summit.
British Prime Minister Tony Blair says there is little doubt that the half-dozen blasts that hit London’s underground metro system and some buses today are terrorist attacks.
"It's reasonably clear that there have been a series of terrorist attacks in London," Blair said. "There are obviously casualties. Both people that have died and people seriously injured, and our thoughts and prayers of course are with the victims and their families."
Blair, who was speaking from the G-8 summit site in Gleneagles, Scotland, also said that Britain would not allow the attacks to derail the summit proceedings.
British Home Secretary Charles Clarke said it is still not known which group is responsible for the attacks. "As yet, we do not know who or which organizations are responsible for these criminal and appalling acts," he said.
A group claiming to represent the Al-Qaeda network claimed responsibility for the attacks in a message posted to an Islamic website. However, the message could not be immediately authenticated.
London Metropolitan Police Commissioner Ian Blair said the details of the attacks remain confused.
"We think there are about six [explosions] so far, but it is still a very confusing scene, and the important point is that we will gradually, but as quickly as we possibly can, assume complete order over the process," Commissioner Blair said.
One witness to an explosion on a metro train described the scene this way "We left the [King's Cross] platform, started going underground, approximately [100 meters], there was an explosion, a flash of light, everything went dark, the train ground to a halt. There was emergency lighting that came on, people started screaming, and then there appeared to be smoke or soot, it was everywhere and it was all over our clothes and our hands and we just had no idea what was going on."
The same witness says the passengers were struck by fear and panic.
"People started screaming," he said. "People felt very uncomfortable. Everyone just spent time with each other, trying to calm everyone down, and that seemed to go fairly well in our carriage, but in other carriages, you could hear people just feeling very uncomfortable and some people grabbed the hammers and started smashing glass, trying to open the door."
London bus drivers are on high alert after today’s blasts. An RFE/RL correspondent in London spoke with one bus driver, who gave his name as Kamal, as he checked his bus for suspicious packages before starting on his route.
“We’re just in an alert [situation]," Kamal said. "As you can see I was checking my bus inside and out, just to know that I am going to take off free [of danger] from here and the minute I get the first passenger, obviously, it is nerve-wracking."
The London blasts suggest some parallels with the terrorist attacks on Madrid’s transportation system on 11 March 2004. As with the Madrid attacks, which occurred three days before a general election, the London blasts appeared timed to coincide with a major political event.
Other European leaders have moved to tighten security in their countries following today’s bombings.
France's government held an emergency session to discuss the attacks. French Interior Minister Nicolas Sarkozy says the government will soon implement "extremely precise" security measures.
In Germany, Berlin city transport authority said it has raised its security level as a precautionary measure.
European Parliament President Josep Borrell expressed condolences to the victims of the attacks. He described the blasts as "barbaric."See also:
Multiple Blasts Create Mayhem In London
Europe Proving To Be Growing Base For Al-Qaeda
Britain Seen As Important Link In Iraqi Jihadist Networks