At 4:25 pm London time, Police issued the following casualty hotline number for people concerned about friends and relatives: +44 (0)870 1566 344. From CNN.com.

Later addition:

People seeking information on Americans believed to have been in London at the time of the attacks urged to call 1-888-407-4747 (toll-free in the U.S.) or 1-202-501-4444 (regular toll line from outside the U.S. and Canada). From MSNBC.com.