For the second time in two weeks, London's public transportation system has come under attack, though police say the latest blasts were much smaller than those of July 7.

London Police Commissioner Ian Blair said there is no evidence that chemical or other unconventional weapons were used, and he has appealed for Londoners to get back on the subway and buses as normal.

London Police Commissioner Ian Blair (APTN photo)

"The situation is absolutely at the moment under control, in as good a position as we could possibly be, given the fact that we have had these four attempts at causing serious explosions in London," he said. "We only have one confirmed casualty and that is not a fatality."

The commissioner said three subway lines - the Victoria, Northern and Hammersmith and City - remained closed late in the day, as the investigation continued.

Prime Minister Tony Blair convened a meeting of his emergency response team to assess the situation. Afterward, he told reporters that the bombers hope to frighten Londoners, and he appealed for calm.

"It is important that we respond by keeping to our normal lives and doing what we want to do, because to do otherwise is to give them the very thing they are looking for," he said.

Eyewitnesses to the incidents report hearing small explosions on the subway, and then smelling what they described as burning insulation or rubber. The driver of the bus involved says the shock of the explosion knocked out windows on the upper deck.

Police and emergency services quickly mobilized to deal with the situation, closing subway stations and stopping buses from circulating. Emergency response teams wearing protective clothing entered the subway, along with police dog units and anti-terrorist branch officers.

Two arrests were reported in the hours just after the blasts, including one man detained one block from Prime Minister Blair's office on Downing Street. It was not immediately clear if the detentions were related to the explosions.

Police have blamed the July 7 bombings on four British Muslim men with apparent links to the al-Qaida terrorist network. A wide-ranging international investigation continues in that case.