London ? In a show of force to calm an edgy London, officers armed with submachine guns patrolled uncrowded subway stations and police helicopters hovered over city streets as one of the busiest Underground lines reopened Thursday – four weeks to the day after suicide bombers killed 52 people.

Undercover officers mixed with a lighter-than-usual load of subway commuters – many Londoners no doubt mindful that terrorists had attempted similar attacks exactly two weeks after the deadly July 7 bombings.

British police investigating the botched London subway attack of July 21 said Thursday they had charged two sisters for failing to disclose information under anti-terror laws.

The city’s nerves were further rattled by a message from al-Qaida warning of more bloodshed, but people made efforts to carry on.

“People didn’t seem frightened or apprehensive, they were just reading their newspapers and getting on with things, just as the English do,” said commuter Pat Wish.

It is clear, however, that the attacks have changed the city.

London’s transport authority said subway ridership – normally 3 million passengers daily – had fallen between 5 percent and 15 percent on weekdays and 30 percent on weekends. Hotels have reported cancelations and large downtown stores say sales are down.

There are signs of fraying in London’s famously diverse social fabric, too. London police have recorded a 600 percent increase in crimes motivated by religious hatred since the suicide bombings.

In a message broadcast Thursday, al-Qaida No. 2 Ayman al-Zawahri linked the attacks to Prime Minister Tony Blair’s decision to send troops to Iraq. “Blair has brought to you destruction in central London, and he will bring more of that, God willing,” al-Zawahri said.

He warned Britain that more destruction lies ahead and promised tens of thousands of U.S. casualties in Iraq.

Blair has strongly rejected any links between the attacks and Britain’s presence in Iraq.