Home Secretary Jacqui Smith has said a car bomb alert in central London showed Britain faced "serious and sustained" threat from international terrorism.
Jacqui Smith had a baptism of fire on her first full day
The new Home Secretary had been expecting to spend the day familiarising herself with her brief.
Instead she chaired the government's Cobra emergency planning committee for the first and updated Cabinet colleagues and the PM on the incident.
She urged the public to "remain vigilant" and assist the police.
Prime Minister Gordon Brown was told about the car bomb alert when he awoke in his Downing Street flat on Friday morning.
The incident overshadowed his second Cabinet meeting, although his official spokesman stressed ministers only spent 15 minutes discussing security during the one hour and 45 minute meeting.
Afterwards, Mr Brown had a private meeting with Ms Smith.
Speaking afterwards, she said: "This latest incident reinforces the need for the public to remain vigilant and alert to the threat that we face at all times.
"I would particularly encourage all members of the public to report to the police anything suspicious.
"While we can minimise the risks, we can never completely eliminate them.
"But as the prime minister has made clear, the government, the police and the security services are doing everything possible to protect the public."
She was accompanied by the UK's new security minister, former first Sea Lord Sir Alan West, who was among five ministerial appointments from outside government announced by Mr Brown as part of his government reshuffle.
A controlled explosion was earlier carried out on the Mercedes, packed with 60 litres of petrol, gas cylinders and nails, in Haymarket, near Piccadilly Circus, central London.
Police sources say if it had exploded it would have caused "carnage".