A crowd of curious onlookers - from men in safety helmets to office workers in snappy suits - gathered at Empress Place yesterday morning.

What had stopped them in their tracks?

Four massive trucks that had trundled over.

They were loaded with four one-of-a-kind BMW racing cars. They had been used as canvases by renowned artists - Australian Ken Done, American Frank Stella, the late Andy Warhol and Roy Lichtenstein.

Moving the cars was a delicate operation, given their value - the BMW organisers did not wish to disclose how much - and rarity (these are the only cars the four artists worked on).

The New Paper team watched as the workers tackled the first car, a 1976 BMW Art Car by Stella.

First, workers had to manoeuvre the car, which weighed about 1,200kg, out of its air-conditioned container truck onto a wooden platform.

The special air-conditioned containers serve to preserve the integrity of the paint from the heat and humidity.

A BMW's Group Mobile Tradition expert, Mr Christian Knegele, turned the steering wheel to adjust the tyres, while four other logistics personnel pushed the car into position.

Next, a 16m high crane lifted the platform - with the car - into a glass and steel structure specially built to house the cars during the three-week BMW Art Cars exhibition.

Since the cars have to pass between a pair of palm trees to enter the glass and steel structure, the operation was a painstaking one.

According to Mr Michael Lim, the regional manager of BMW Group Asia, it had to be done 'very carefully, very gingerly and with a lot of prayers'.

Every detail of the operation was looked into.

Rubber non-slip grids were used to hold the cars in place within the container, said Mr Lim.

To keep out moisture and dust, special claddings are drapped over the cars when they are not on display.

The people handling the car were not allowed to have jewellery on their hands and they had to wear gloves, said Mr Lim.

Chemicals cannot be used to clean the cars, only a special soft cotton cloth is used to wipe off dust from the car.

Up to two BMW Group Mobile Tradition experts tag along with the cars on their global tour.

And they are the only ones allowed to lay their hands on the the cars' interiors.

'Nobody in BMW can touch the cars, only the experts are able to as they are trained to handle the cars with care,' said Mr Lim.

BMW's global exhibition - which costs $35 million - has been to museums like the Louvre in Paris, the Royal Academy in London and the New York Whitney Museum of Modern Art.

The BMW Art Cars exhibition is on at Empress Place, beside the Asian Civilisation Museum, from 24 Jan to 11 Feb, from 12pm to 5pm weekdays and 9am to 9pm weekends. Admission is free.