LONDON, United Kingdom (AFP) - Flags were flown at half mast above Scottish government buildings on Tuesday to remember six people killed when an out-of-control lorry ploughed into Christmas shoppers in Glasgow.

Another 10 people were injured, including the driver, after the refuse truck swept along a crowded pavement in the city centre on Monday afternoon, knocking pedestrians down "like pinballs", according to one horrified eyewitness.

The Church of Scotland organised a prayer service for the victims on Tuesday morning and opened a book of condolences at St George's Tron church, a short walk from the scene of the tragedy on George Square.

Floral tributes were placed at the scene of the crash, which police said appeared to have been a tragic accident. The dead included five women and one man, police said.

The city returned to mourning just over a year after a police helicopter crashed into a Glasgow bar on Nov 29, 2013, killing eight people and injuring more than 30 others.

First Minister Nicola Sturgeon described Monday's incident as "another sad day for Glasgow and Scotland".

"This morning it's a city with a broken heart but it will get through this as it got through the Clutha (helicopter) tragedy," she told BBC radio.

The local authority bin lorry veered over a distance of around 300metres, only stopping when it crashed into a hotel near Queen Street station.

The driver survived and was taken to hospital. Eyewitness reports suggested he was slumped at wheel, raising speculation that he may have had a heart attack.

An investigation was under way to establish exactly what had happened, police said.