Several near simultaneous explosions on Thursday rocked the British capital, causing many casualties and at least 37 deaths.
At the G8 summit in Scotland, leaders of eight of the world's most industrialised nations described the acts as "barbaric".
Swiss President Samuel Schmid expressed his solidarity with all those affected by the attacks.
He said the events in London showed that attacks on civilians could occur at any moment and 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 Swiss government also expressed solidarity with the British people in a statement. It said 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."
Police said there were three explosions on the Underground which left 33 dead and one on a double-decker bus in which an unknown number died. More than 345 are estimated to have been injured.
The explosions, which came one day after London was awarded the 2012 Olympics, led officials to shut down the entire bus and underground transport network. Some stations have now been reopened.
Television pictures showed a mangled red bus with its top collapsed, and police said they suspected a bomb caused the explosion.
Bloodied and bandaged witnesses reported panicked crowds fleeing as the blast rained glass and other debris onto the street.
London police chief Ian Blair said authorities found indications of explosives at the scene of one of the blasts. He said there had been no warning.
A building near London’s Liverpool Street station belonging to Switzerland’s largest bank, UBS, was evacuated after the blasts.
A spokesman for the bank could not say how many employees were affected but confirmed that trading had not been suspended.
Elsewhere in Europe, Germany’s national railway operator took steps to bolster security and Berlin transport officials raised the alert level to "yellow".
The explosions sent stocks plummeting in Europe, with investors buying safe-haven Swiss francs, gold and government bonds.
The Swiss franc hit a six-week high against the British pound and rose more than one per cent against the dollar. It was marginally higher against the euro.
At its lowest point on Thursday, the blue-chip Swiss Market Index dropped 3.1 per cent to 6,113 points. London’s FTSE 100 was at one point down 3.2 per cent at 5,062.
The pan-European virt-x bourse, where Switzerland's blue-chip stocks are traded, announced that it was transferring trading from London to Zurich until further notice.
Virt-x is a wholly-owned subsidiary of Switzerland's SWX group.
Transport and tourism operators’ shares were hardest hit, along with insurers. Swiss reinsurers Swiss Re and Converium saw their shares drop 4.3 per cent, while stocks of travel company, Kuoni, fell 3.8 per cent.
The Swiss franc firmed against rival currencies, trading at around SFr1.295 against the dollar as investors sought out its safe-haven credentials.
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.