WALL STREET was on alert yesterday as traders and bankers arrived at their desks. Sniffer dogs and police armed with machine guns filed slowly up and down in front of the New York Stock Exchange, checking vehicles, doorways and rubbish bins for anything out of the ordinary.

Four vehicles from the New York City Fire Department screamed on to Broad Street, behind the NYSE, moments before trading began, to perform security tests. Police were stopping cars and trucks in the surrounding streets, asking drivers to turn off and turn on their engines to prove that they had a working battery.

Security sources said that such a test is a simple method to detect car bombs.

Despite the heightened security, the NYSE said that it was “business as usual” on Wall Street.

John Thain, chief executive, offered his condolences to the people of London. “The President of the United States spoke for all Americans in expressing our condolences and support for the people of London,” Mr Thain’s spokesman said.

In Times Square, tourists gathered outside the Nasdaq market to watch the events unfold in London on a giant TV screen. Security in Manhattan’s busiest thoroughfare was also heightened, with armed police and dogs surrounding Nasdaq and many of the area’s big shops.

Just before 10am, Nasdaq showed a giant Union Jack and a message of condolence on its electronic message board, high above Times Square.

The blasts coincided with research by Tobias Levkovich, the chief US equities strategist at Smith Barney, entitled Cataclysm, Terrorism and Stocks. The research claimed that markets are recovering increasingly quickly after terrorist attacks.

“On a human level I am appalled by this attack,” Mr Levkovich told The Times. “But in a cold analytical way we can see quite clearly that markets have become quite immune to events such as this.”