SINGAPORE - As a siren pierced the quiet at 8am, some 400 soldiers at Selarang Camp in Loyang rushed to fall in at the parade square with their weapons, field packs and helmets.

The troops, from the 3rd Battalion Singapore Infantry Regiment (3SIR), were on standby. They checked their mission-essential items and performed weapon drills to prepare for a possible operation at night.

As news came later in the evening of multiple simulated terror attacks around the island, they were deployed at one of these locations - the Sports Hub - to guard against armed "perpetrators" who, under the exercise scenario, disrupted a game at the National Stadium.

The following afternoon, thousands more operationally-ready national servicemen (NSmen) were activated as part of an effort to test the army's ability to mount a sizeable force within hours.

The series of events over two days unfolded as part of the Singapore Armed Forces' largest mobilisation exercise since 1985, and the first one to feature a homeland security operation with the police.

A total of 8,000 troops and 700 vehicles from the 9th Singapore Division responded to the call of duty for the exercise that ended on Saturday (Jan 27).

A new enhanced Mobilisation and Equipping Centre (MEC) at Selarang Camp, a one-stop building for recalled servicemen to draw their combat gear completed last year, was tested for the first time in the exercise.

The four-storey complex, taking up one-third of Selarang Camp, introduced features such as self-service kiosks for servicemen to register at the camp, reducing the time taken and making the process of getting equipped more efficient and less resource-intensive.

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Speaking to the media near the end of the two-day exercise, Defence Minister Ng Eng Hen said: "This (mobilisation) included an anti-terrorism exercise that we did together with the police force. So whether it's for humanitarian action, low-intensity conflict or terrorism threat, or even in the unthinkable scenario of outright war, I think Singaporeans have that confidence that the army is ready, and NSmen are committed and ready to be equipped, to be able to fulfil their mission."

At the Sports Hub, troops from 3SIR conducted vehicle and personnel access control operations against armed gunmen, ensuring that the "spectators" were evacuated in an orderly manner but that the "perpetrators" did not escape by blending into the crowd.

The operations were mounted together with Singapore Police Force officers and Sports Hub security officers.

Brigadier-General Ng Ying Thong, 45, who is the army's assistant chief of the general staff for operations, was present for the exercise on Saturday morning.

He said: "This time round, we bring the troops out to a very realistic and iconic location here at Sports Hub. I think this will further reinforce their commitment and conviction to make sure Singapore is safe and secure."

Mobilisation exercises were introduced in the 1980s to test the operational readiness of its reservist forces.

The SAF conducts about 20 to 30 such exercises a year, with an average attendance of 95 per cent for each exercise.

Second-Lieutenant Syahir Ally Sa'ari, 25, a platoon commander in 3SIR, said the training at the Sports Hub was realistic as the scenarios given were unexpected.

"For example, for the vehicle that came in, we did the normal checks, and one of my troopers found an explosive. That shows how well trained our troopers are to execute what they have learnt," he added.