The all-weather cameras, which keep watch on streets and other public areas 24 hours a day, have deterred crime, police said.

An additional 223 closed-circuit television (CCTV) cameras could join more than 100 electronic eyes already in use.

The orb-shaped cameras, mounted on buildings or standalone poles, continually record street scenes. The 'live' images are sent to a command monitoring centre, which is manned day and night.

Police have issued a 'request for information'' (RFI) to vendors.

When extended, the CCTV network looks set for a massive overhaul. Not only will more cameras be fitted, but images from various parts of the island will be linked to build a seamless 'big picture' of the prevailing security situation.

The RFI document asked suppliers for ideas to integrate footage from the CCTV network with images filmed at MRT train stations and immigration checkpoints at Tuas and Woodlands.

Security cameras first sprouted in 2003 in tourist spots like Boat Quay and Orchard Road, in the Shenton Way financial district and in Little India and Geylang.

Police spokesman Mohamed Razif said the locations for the new cameras are being assessed.

He said: 'CCTVs have long been recognised for their psychological deterrent effect on potential offenders and have been used to monitor premises as part of crime prevention and security measures.

'Where crimes have occurred in the vicinity of CCTV cameras, the recorded footage has proved useful in police investigations and eventual arrests of criminals.''

Indeed, CCTV cameras helped the cops crack a 2004 case at Newton Food Centre when a group of 10 men armed with parangs, samurai swords, knives and metal rods attacked two men.

Police nailed the assailants after clothing found in an abandoned van matched that worn by the assailants in the footage of the assault.

'Such breakthroughs and successes have upped the deterrence effect against criminals, who are more wary of committing crimes in areas where there is CCTV coverage,'' Inspector Razif added.

Ms Ng Sue Chia, an associate research fellow at the Centre of Excellence for National Security, S. Rajaratnam School of International Studies, Nanyang Technological University, said experience elsewhere attests to the cameras' effectiveness.

But it was important to balance crime prevention with the public's right to privacy, she added.

Even with the additional cameras, Singapore's streets will not be as closely watched as London's, where a person could be caught on CCTV about 300 times a day.

Said Ms Ng: 'There is a difference between a state that is well-policed and one where every action is monitored.

'There would certainly be some initial discomfort over the increased presence of CCTV cameras. Public awareness campaigns would both help to inform and mentally prepare the public in accepting the need for increased security.'

dboey@sph.com.sg