The incident occurred at 0751 GMT between Liverpool Street and Aldgate tube
stations, the City of London's press office said.
"The entire tube network is closed," said Lesley Mair, press officer for the
City of London.
At least some people have been wounded, but no numbers were yet available,
Mair said.
"The original call we got this morning was to the Liverpool Street station,"
said Allison Kerry, communications officer for the London Ambulance Service.
"Since then, there are other locations that we are in attendance. We're not
clear at this stage as to what's caused this. We've sent out lots of
ambulances to various central London locations, and we're working with other
London emergency services," she said.