This post will be updated throughout the day.
Three explosions rocked the London tubes this morning and one tore open a packed double-decker bus during today's rush hour, sending victims fleeing after what Tony Blair called "barbaric" terrorist attacks. London Mayor Ken Livingstone, in Singapore where he supported London's successful bid to host the 2012 Olympics, said the blasts that ripped through his city were "mass murder" carried out by terrorists bent on "indiscriminate slaughter." (A group called The Secret Organisation of al-Qaeda in Europe said it carried out the series of blasts in retaliation for Britain's involvement in Iraq and Afghanistan.)
There are conflicting reports over how many people were killed and injured but a US law enforcement official is being widely quoted saying that at least 40 people were killed and London hospitals have reported more than 700 wounded. Whatever the exact numbers, it's clear that this is Britain's worst-ever terrorist attack.
Click here to read The Guardian's continuing coverage and watch the The Nation online for further reporting on events.
(If you need information on someone who may have been injured in the attack, the British police have listed this emergency hotline number: 44-0870-1566-344)