Speaking outside 10 Downing Street, Mr Clarke confirmed that the COBRA civil emergencies committee of senior ministers had met.
The police had taken "operational command" of the situation, he said.
"Health services are in support to deal with the terrible injuries that there have been and I want to express sympathy on behalf of the whole Government," he said.
Prime Minister Tony Blair, who is at Gleneagles for the G8 summit, will make a statement soon.
London Mayor Ken Livingstone is understood to be returning to London from Singapore, where he had been for the Olympics announcement.
- More about: