Switzerland has joined other countries in condemning a series of bomb attacks in the British capital, London, on Thursday.
On Friday afternoon, the official death toll from the four blasts stood at 50, with more than 700 wounded.
Police said that there had been three explosions on the London Underground and another blast on a double-decker bus which had left 50 dead.
More than 700 people are estimated to have been injured in the attacks.
"We are treating this as a terrorist incident," said Brian Paddick, deputy assistant commissioner of the Metropolitan Police, earlier on Thursday, adding that no warning had been received.
Paddick said there had been no arrests so far and it was unclear if suicide bombers were involved. Police are examining claims that an organisation linked to the Islamic militant group al-Qaeda was behind the bombings.
Blair said it was "reasonably clear" that the attacks had links to terrorism. He has now returned to the G8 summit in Gleneagles in Scotland, which he is hosting, after visiting London.
Speaking before he left, he condemned the attacks and promised "intense action" to bring those responsible to justice.
Meanwhile, the British foreign secretary, Jack Straw, said the bombings had "the hallmarks of an al-Qaeda-related attack".
The explosions have provoked worldwide outrage. The heads of the world’s leading industrialised nations attending the G8 summit, United Nations Secretary-General Kofi Annan and other countries condemned the acts in a joint statement.
Speaking to reporters in Gleneagles, President Bush said "the war on terror goes on".
Both Swiss President Samuel Schmid and the Swiss government have expressed solidarity with all those affected by the attacks.
Schmid said the events in London showed that attacks on civilians could occur at any moment. He claimed that Britain and other western nations had been expecting an incident of this nature for some time.
"All the security measures taken in Switzerland are unfortunately more than justified," he said, adding that it was no longer possible to "sleep soundly at night".
Schmid said the attacks were against systems and not individuals. He pointed to the fact that Britain and Switzerland were both banking and financial centres, and that British or American firms in Switzerland were potential terrorist targets.
The government said in a statement that relatives of Swiss citizens who might be affected would be informed as quickly as possible.
Ivo Sieber, spokesman for the foreign ministry, told swissinfo: "We’re in contact with the Swiss embassy in London and are busy clarifying whether there are any Swiss victims."
Security is reported to have been tightened outside the British embassy in Bern. Britain's ambassador to Switzerland, Simon Featherstone, thanked the Swiss authorities for their prompt reaction and said a hotline had been put in place for worried relatives.
The Federal Police Office declined to comment on whether protection was being stepped up at other foreign missions, but said it was monitoring the situation closely.
The explosions, which came one day after London was awarded the 2012 Olympics, saw officials shut down the entire bus and underground transport networks. Some stations have now been reopened.
A building near London’s Liverpool Street station belonging to Switzerland’s largest bank, UBS, was evacuated after the blasts.
The explosions sent stocks plummeting in Europe, with investors buying safe-haven Swiss francs, gold and government bonds.
swissinfo with agencies
A number of major Swiss financial institutions, including UBS and Credit Suisse, have offices in the city of London.
The bank, UBS, evacuated a building in the city after the bombings.
There are about 25,000 Swiss citizens living in Britain.
The Swiss foreign ministry is trying to establish whether any Swiss have been caught up in the blasts.
A GMT timeline of the events in London:
0751: Explosion on underground train near Liverpool Street station.
0756: Explosion hits train on the Piccadilly underground line between King's Cross station and Russell Square.
0817: Explosion on circle line train going into Edgware Road underground station.
0847: Explosion rips through a double-decker bus near Tavistock Place.