Two people have been killed and scores have been injured after at least seven blasts on the Underground network and a double-decker bus in London.
UK Prime Minister Tony Blair said it was "reasonably clear" there had been a series of terrorist attacks.
He said it was "particularly barbaric" that it was timed to coincide with the G8 summit. He is returning to London.
An Islamist website has posted a statement - purportedly from al-Qaeda - claiming it was behind the attacks.
Walking wounded (BBC photo)
Our prayers are for the people of London and England.
Update: Excellent coverage all around the blogs. Rantingprofs has a guest blogger with excellent posts here and here.