Many Underground trains and buses on Friday cautiously resumed operations in London [Images], a day after Islamic militants carried out coordinated attacks in the British capital, killing 37 people and injuring 700.

There was heavy security presence as subway lines reopened and reluctant commuters slowly resumed travel saying they had "little choice."

Tube stations opened at their normal time, although delays were expected due to partially or fully closed underground routes. Two of the lines affected by the blasts were yet to open, officials said.

Bus service also resumed in central London, except for diversions around blast sites.

Officials said tracker dogs and extra police had been deputed to oversee passenger safety as the network slowly began to return to normal.

Even as a massive hunt was underway to track the bombers, who struck at three subway cars and a double-decker bus, the flag at Buckingham Palace flew at half-mast to mourn those who perished in the terror attacks.

Security was tightened at the Heathrow, the world's busiest airport as discussions continued at the G8 Summit in Gleneagles.

British Prime Minister Tony Blair [Images], determined not to be deterred by the deadly attacks, returned to the G8 summit he had left in Scotland in the aftermath of the serial attacks.