As part of its remit to keep the city-state "safe and sovereign" the Internal Security Department (ISD) conducts one of the world's most unusual public-awareness campaigns. Under the umbrella of an education programme, ISD operatives have for at least the past five years ventured into the public arena to spread the official line on how citizens should understand the world around them. The efforts are carefully targeted, with briefings for those perceived to wield the most influence.

An ISD spokesman says the agency's aim is to "sensitise" its chosen audience in a bid to ensure no one has any "misconceptions on security issues". The spy-catchers' language is clinical, with echoes of the tone preferred by the now-defunct Communist governments of Eastern Europe.

"The security education programmes of the ISD sensitise key Singaporean constituencies, mainly in the public sector, on fundamental security issues," the spokesman says. "These include briefings to government ministries [or] departments and grassroots leaders, protective security training to security personnel in the public service, symposiums and seminars on counter-terrorism and racial and religious extremism as well as information technology security seminars."

Although the programme is rare, its breadth makes it even more so. Official figures show that ISD staff have met 45,000 people since the mid-1990s. As Singapore has a little over three million citizens, many of whom are too young or too old to be of much interest to the ISD, the coverage appears remarkable.

The ISD traces its roots back to the beginning of the 20th century when it was used to bolster British colonial rule. Its forerunner was the police special branch, which was set up in 1919 to deal with "political subversion". With the turbulent times, the body thrived.

In 1945, the Malayan Security Service was set up, with Singapore as its headquarters. Three years later, an administrative revamp saw the creation of a new unit - also called the special branch - which evolved into today's ISD. During Singapore's ill-fated attempt to chart a course as part of the Malaysian Federation from 1963 to 1965, the agency fell under the jurisdiction of Kuala Lumpur. After independence, it was placed under Singapore's interior and defence ministry, and in 1966 given the name it carries today.

The ISD's curt official history shows that although its principal activities may have evolved over time, its guiding ethos has remained constant. Quite understandably for a tiny, majority ethnic-Chinese country in the midst of a sometimes hostile Malay-Muslim sea, Singaporean leaders have an acute sense of their state's vulnerability. The ISD is one of several agencies - along with the armed forces and the Security and Intelligence Division, which deals with overseas intelligence - charged with safeguarding Singapore's continued existence.

The ISD's rallying call comes from the country's founding father Lee Kuan Yew, now Senior Minister. "Stability does not come naturally to Singapore. We are peculiarly vulnerable," says a quote from Mr Lee used in ISD material. "If our balance of security and stability is shattered, it is doubtful if we can on our own ever put Singapore together again."

Guided by that warning, ISD has been on guard for the past three and half decades. In the 1960s and 1970s, the principal fight was against the remnants of the Communist Party of Malaya (CPM), and the threat of violent communalism. The agency credits itself with having prevented "a bloodbath" in Singapore after the Chinese-Malay riots that hit Malaysia in 1969.

In 1974, it helped to end the Japanese Red Army's hijacking of a Singapore ferry, the first terrorist incident in the country's modern history. A decade later, there was another high-profile result. "In 1982, the ISD exposed and expelled two Russian spies for espionage in Singapore," it says. "There are many cases involving other countries. However, for national security and other reasons, they are not made public."

Events that did make it into the public arena - but not ones the ISD chooses to publicise - were a swathe of detentions of suspected "Marxists" in the late 1980s.

"We collect accurate intelligence, make impartial assessments and take timely action to counter threats to Singapore's internal security and sovereignty," the agency says. "We do this without fear or favour, because we passionately believe that Singapore belongs to all its citizens regardless of colour or creed, and that for a small, multi-racial city-state, our collective survival hinges on continued stability and security."

These days it appears to be quieter on Singapore's domestic front, allowing the ISD to embark on its role of public educator alongside its more usual fare of counter-espionage and vetting of public-sector employees. "ISD is a major participant in the government's continuing process of establishing the national framework," the ISD says, although it remains vague about what that term means. What is more certain is that the ISD has stepped up its work after the September 11 terrorist attacks on the United States. Along with heightened vigilance against foreign groups who may be intent on destabilising Singapore, there is also great concern that the "war against terrorism" should not undermine race relations.

Singapore has a small Malay-Muslim community - about 10 per cent of the population - and politicians have urged them not to be swayed by military action in Central Asia.

The ISD spokesman says the department "has been giving security briefings to many different groups after the September 11 incident, to sensitise them to the threat, and to urge them to be vigilant." Suspected terrorist mastermind Osama bin Laden, the chief suspect behind the attacks, has helped to draw the ISD a little further out of the shadows.