British police suspect that four British-born men, at least three of them of Pakistani origin, were the bombers and say all of them might have died in the four separate bomb blasts on public transport.
The attack could mark the first time that suicide bombers have been used in Islamic extremist attacks in the West.
Such bombers can mingle practically undetectable in large crowds.That has heightened the nervousness in other European capitals, particularly Paris, which today celebrates Bastille Day, France's national holiday, with a big military parade along the Champs Elysee and a speech by President Jacques Chirac.
France has increased security and taken back temporary control of its own borders from the EU's Schengen open border system, in order to hamper possible movements by terrorists.
Thierry Balzac, a security analyst at the Center for European Policy Studies in Brussels, said the big terror strike in London reminds Europeans yet again of the danger they face.
"Now that this thing in London has happened, it actually reminds people of [the March 2004 bombings] which took place already in Madrid, so Europe now feels much more vulnerable," Balzac said.
The Madrid and London attacks are only the tip of the iceberg, according to Balzac. He noted police and security agencies on the European Continent and Britain claim to have foiled many attacks. London and Madrid were those which managed to slip through the net.
Italy has also sought to grasp the initiative to ward off possible attacks. Italian security forces have made a nationwide security sweep against suspected Islamic extremists.
Hundreds of police searched Italy's Islamic and North African communities, raiding homes and seizing materials in Rome, Naples, Turin, Florence, Bologna, Siracusa, Ragusa, and other cities.
Italian officials admitted they are worried that Italy could now be on the terrorist hit list, following London. Italy has been one of the strongest supporters of the U.S.-led invasion of Iraq after Britain --- but unlike Paris, which has opposed the war.
There is also quicker-moving antiterror action at the European level. In Brussels, senior EU ministers met in an emergency session yesterday to discuss how to respond to the London bombings. The ministers agreed to speed up a package of antiterror measures on cutting off terrorists' finances and increasing cooperation between intelligence services in the EU.
British Home Secretary Charles Clarke, whose country currently holds the rotating EU Presidency, said the union is "determined" to get a better grip on controlling terrorists' activities.
"All of us across the European Union are absolutely determined to accelerate our work to make terrorism more difficult," Clarke said. "It [the agreement] focuses around a wide range of different exchanges of data and information, whether on stolen explosives, on communications data, [or] on operational cooperation between different forces."
The present lack of coordination between the European partners is illustrated by a misunderstanding between Clarke and French Interior Minister Nicolas Sarkozy. Sarkozy said Clarke had told him that some of the suspects in the London bombings had been arrested previously. Clarke denied he had said that.
British police sources have suggested the four suspects had no police records, which would have made them less "visible" to police and intelligence services.
Analyst Balzac, however, said he doubts that administrative or police methods can be entirely effective in suppressing terrorism. The central question today, he said, is why young Muslims in Europe are apparently increasingly willing to resort to violence on a grand scale?
He said he does not believe the Western intervention in Iraq is not sufficient grounds for this in itself.
"Iraq can be seen, so to say, as something intensifying or accelerating the process of frustration among some young European citizens of different origins," Balzac said.
Human rights groups have expressed concern that EU action to create or strengthen antiterrorism laws could interfere with the civil rights of citizens. Britain has gone furthest with its laws, and Home Secretary Clarke yesterday called on the European Union to overcome civil liberties concerns and agree on new antiterrorism measures. Clarke told the European Parliament's Committee on Civil Liberties that living without the fear of terrorist attacks should be a recognized human right.
EU Justice and Home Affairs Commissioner Franco Frattini appeared to address these concerns yesterday when he spoke of the need for balance between rights and restrictions.
"Another important point is to balance, to keep a balanced approach between fundamental rights and liberties and the right to security that is in itself a fundamental right," Frattini said. "If I may say, it is a precondition to enjoying all fundamental liberties and rights."