Fear Injection

LONDON, England (CNN) -- Four small explosions have hit three London Underground stations and a bus two weeks after the July 7 terror attacks, the city's police chief has said.

Thursday's lunch-hour attacks in the UK capital came two weeks to the day after morning rush-hour bombs on three Tube trains and a bus killed 52 people and the four bombers.

Metropolitan Police Commissioner Ian Blair later told said the situation was "absolutely under control." He told reporters there had been "one confirmed casualty and that is not a fatality."

"We know that we've had four explosions or attempts at explosions. It is still pretty unclear as to what's happened.

"The bombs appear to be smaller than on the last occasion but we don't know the implications of all this yet and we're going to have to examine the scene very carefully…

Scotland Yard said initial examinations at all four sites -- Warren Street, Oval and Shepherd's Bush stations and a bus in Hackney -- showed no trace of chemical agents.

Police also said they deployed armed officers to investigate an "incident" at University College Hospital, near Warren Street, where an explosive device went off aboard a train…

A Scotland Yard spokesperson said two arrests were made in the Westminster area but said they were not necessarily linked to the attacks.

At a news conference, Prime Minister Tony Blair praised the police operation and urged Londoners to remain calm.

"We know why these things are done. They're done to scare people and to frighten them, to make them anxious and worried," he said.

Blair, who referred questions on operations to police and emergency services, told reporters: "Police and security services are pretty clear about what's happened.

"The police have done their very best and the security services, too, in this situation. And I think we just have got to react calmly and continue with our business as much as possible as normal."