European governments have united in condemning the string of London attacks, as some stepped up security levels in their own countries.
Italy has raised security levels
France, Spain, the Netherlands and Italy have raised national levels of security, while Belgium has stepped up its precautions.
European Union Justice Commissioner Franco Frattini called on Europe to offer the UK all the help it needed.
Security is being tightened at airports and railway stations across Europe.
Other countries stepping up security measures include Russia, the Czech Republic, Hungary and Romania.
Following the attacks, Belgian authorities met to discuss security measures in Brussels, which is also home to a number of EU institutions and Nato.
There was also a heavy police presence in Spain
Belgian Prime Minister Guy Verhofstadt later said there was no evidence that Belgium was at a higher risk, but extra security precautions would be taken on trains and subway stations and at border checkpoints, airports and seaports.
Spain - the target of terror attacks which killed 191 people in Madrid on 11 March last year - raised its security standing to the highest level of anti-terrorist alert.
"This level of alert involves the absolute mobilisation of the security forces to keep watch and protect places with large amounts of people as well as strategic sites needed for normal citizen activity," the interior ministry said.
The Spanish government offered to help the UK "pursue the criminals who have carried out such a repugnant attack against a city that was celebrating its election as the host of the 2012 Olympic Games".
French Prime Minister Dominique de Villepin announced France's step up to red alert - the second-highest level of security.
Security is especially tight for rail passengers travelling between Paris and London. Eurostar trains are still running but the company has advised people not to travel to London.
Italian Interior Minister Giuseppe Pisanu told the Senate: "The level of alert has been raised in Italy as in all the other European countries."
Dutch Prime Minister Jan Peter Balkenende said security would be stepped up at the Dutch borders and around British sites in the Netherlands.
He declined to give further details about the potential targets being given added protection.
In Paris, soldiers patrolled train stations
Other countries paid tribute to the victims of the attacks.
The French Senate suspended its session "out of respect" for the victims and the European Parliament held a minute's silence.
French President Jacques Chirac, at the G8 summit, told UK Prime Minister Tony Blair of his horror at the attacks and expressed France's solidarity. The Russian government also joined the condemnation of the attacks.
Speaking in Rome, Mr Frattini said the events in London were "a tragic confirmation that terrorism strikes once more at the heart of Europe".
"It's necessary to immediately activate co-ordination between intelligence and police services and offer England all the help possible," he said.
European Parliament President Josep Borrell, who is Spanish, expressed condolences on behalf of the whole parliament "to all suffering consequences of these barbaric attacks".
"As president of parliament and a citizen of a country that only last year experienced the horrors of such terrible attacks, I want to send a message of solidarity with British people," he said.
"We all stand with you, British people, and we will never let atrocities or terrorism defeat the values of peace and democracy."