The "ring of steel" which surrounds key sites in the City of London has been widened.
Armed police are manning checkpoints in the City
The new zone, between Holborn and Victoria Embankment, covers businesses employing 45,000 people.
Concrete blocks narrowing the road, police check points and surveillance cameras have been set up.
The security measures in London's financial district were first introduced in the mid-1990s during an IRA bombing campaign.
They were set up after the Bishopsgate attack in 1993 which killed one person and injured 40, causing more than £300m in damage.
The Corporation of London, which runs the City, said the decision to strengthen security was not in response to any specific threat.
James Hart, Commissioner for the City of London Police, said: "As well as the traffic and environment benefits which we welcome, the zone will assist the force in its on-going efforts to deter terrorism and other criminality."
The measures include automated number plate recognition cameras at entry points - similar to those used in London's congestion charge scheme.
Visible security measures have been heightened in central London since the terror attacks in the United States on 11 September 2001.
Armed police regularly patrol areas popular with shoppers and tourists and concrete blocks have been placed in front of high-profile buildings to deter car and truck bomb attacks.
Last month a study found that London is at greater risk of a terrorist attack by Islamic extremists than any major city in the US or Western Europe.
Real IRA attacks
The Control Risks Group said the UK's role in the Iraq war and big Muslim population meant there was a "serious possibility" of a suicide bomb.
The last terrorist attacks in London were in 2001 when Real IRA bombs went off in west London outside the BBC TV headquarters in March and in Ealing Broadway in August.
They caused millions of pounds in damage but no-one was killed, although there were some injuries at Ealing Broadway.
Five men were jailed for those attacks at the Old Bailey in March this year.