Car horns and sirens sounded, church bells rang out and roads were crowded with vehicles as Samoa today became the first country in decades officially to switch from right- to left-side driving.

Officials urged the public to stay vigilant, warning that the danger of crashes could increase in coming days if drivers became complacent before the new road rules became second nature to them.

After months of preparation, the early-morning switch from right to left went without incident and in an atmosphere of national celebration.

As the 6am deadline approached, the police minister, Toleafoa Faafisi, went on national radio to tell drivers everywhere to stop their vehicles. Minutes later, the prime minister, Tuilaepa Sailele Malielegaoi, broadcast the formal instruction for drivers to switch sides.

While there was some hesitation when drivers resumed their journeys, traffic soon flowed again with guidance from police as hundreds of onlookers who lined streets in the capital, Apia, clapped and cheered.

"All we want to see is how smooth it is and how safe it is, and I think we have seen that this morning," one unidentified Apia resident told Television New Zealand. "I mean ... you see how cars are moving now, so to me it's good."

The government made the change to bring Samoa in line with Australia and New Zealand, where some 170,000 expatriate Samoans live. It is cheaper to import cars from there than from right-side-driving countries such as the US.

For now, Samoa will allow cars with steering wheels on either the left or the right side of the vehicle – but all will drive on the left side of the road.

Critics accused the government of failing adequately to prepare drivers, and had predicted traffic chaos and a rise in fatal wrecks.

The switch was ushered in with a two-day national holiday, to keep a limit on traffic, and a three-day ban on alcohol sales, to deter accidents.

Police staffed scores of checkpoints, and warned drivers to keep their speed down.

Leasi Galuvao, the chief executive of the Land Transport Authority, said the safety messages would continue for three months.

"The time of most concern is when motorists believe themselves adjusted to the new conditions and press the accelerator a little harder," he said.

Tuilaepa said the government had already widened roads, added new road markings and signs and installed traffic-slowing speed humps on key roads on the nation's two main islands, Upolu and Savai'i.

Tuilaepa said he was glad Samoa had finally made the switch, despite much resistance and a postponement of more than a year.

Samoa is the first country in decades to change the direction of traffic. Iceland and Sweden did it in the 1960s, and Nigeria, Ghana and Yemen did it in the 1970s.

The issue of buses with both doors and steering now on the wrong side had yet to be addressed, Tuilaepa said.