Six people have been killed and eight others injured after an out-of-control bin lorry crashed into pedestrians in central Glasgow.
The council vehicle then crashed into the Millennium Hotel, near Queen Street station in George Square, at 14:30.
Eyewitnesses described emergency services battling to save the lives of injured people on the street.
Police Scotland has declared a major incident but said there appeared to be nothing "sinister" about the crash.
The force has launched a dedicated number - 01786 289070 - for anyone who has concerns about relatives or friends. It has also urged people in the city centre to let relatives know they are safe and well.
A major investigation is now under way to establish the circumstances surrounding the crash.
Officers have also asked for anyone with photographs, videos or any type of mobile footage of the incident to contact them, using a dedicated email address.
Glasgow City Council offered to turn off the Christmas Lights in George Square as a mark of respect but police requested they be kept on to assist with their inquiry.
Police said the bin lorry was travelling north on Queen Street, outside the Gallery of Modern Art, when "it was in a collision with a pedestrian".
It then continued north on Queen Street, mounted the pavement and struck a number of other pedestrians, before crashing into the Millennium Hotel at George Square. The square is the scene of Glasgow's annual Christmas festival.
BBC Scotland's Laura Bicker said: "What we have heard from eyewitnesses is that the bin lorry appears to have careered out of control and hit a number of people.
"We've also heard from one witness who described the driver of the bin lorry as 'slumped over the wheel'."
Eyewitness Stephen McNeil said: "I was coming out of a shop and we looked over to the left and we saw this woman screaming and this man on the floor.
"We looked around the road further and we seen this bin lorry just mounting the kerb going left to right just hitting people where they were standing. Such a horrible scene."
Witness Janey Godley told the BBC News Channel that people were "getting CPR (cardiopulmonary resuscitation) on the ground" when she arrived in the square.
"I think I was there just as the ambulances arrived, because there was stuff strewn, debris strewn, all over the street, Christmas shopping all over the street," she said.
Anjan Luthra witnessed the crash. He told BBC News: "I work in George Square and had nipped out for lunch, and was in the queue in Greggs. All of a sudden I heard this screaming and quite a consistent racket.
"I ran out of the store and right directly across from me, I was just watching a very very heavy bin lorry just flatten bodies on the pavement. It was driving along the pavement and taking everything along with it, and then it went across the street.
"Immediately people started calling 999, and started directing traffic.
"There were two bodies right in the centre of the T-junction who were getting CPR mouth-to-mouth and there were some bodies that were so fatally injured that you couldn't really do anything."
A Police Scotland spokesman said: "About 1430 hours today, Monday 22 December 2014, a Glasgow City Council bin lorry was travelling north on Queen Street when it appeared to strike pedestrians outside the Gallery of Modern Art before continuing to travel towards George Square, when it crashed at the Millennium Hotel on Queen Street at George Square.
"There have been a number of fatalities and people injured. Emergency services are currently at the scene. Police have declared the scene a major incident.
"All roads in the surrounding area are closed and people are advised to stay away from George Square and make alternative travel arrangements."
Police Scotland later said the crash was "not a terrorism-related incident".
Supt Stewart Carle of Police Scotland said: "With all the drama, people may be alarmed and believed it to be something other than what we believe it to be.
"It is a road traffic collision with multiple fatalities. It is a terrible incident but we don't believe there is anything at this juncture that is sinister about the accident."
Supt Carle said there was a "multiple agency response" from fire, ambulance and police, as well as Glasgow City Council and other support agencies.
He said "the seriously injured and the injured have been taken to the three Glasgow major incident sites".
"We've declared a major incident and we're working with our partners as quickly as possible to resolve the situation and to identify those who have lost their lives and to be there to support their families and next of kin."
Supt Carle told how police "will be here now for many hours and throughout the night as we investigate the scene and eventually as we recover the dead".
All casualties, including the driver of the bin lorry, have been taken to the Southern General Hospital, Glasgow Royal Infirmary and the Western Infirmary.
First Minister Nicola Sturgeon said: "My thoughts are with everyone involved in the dreadful George Square incident."
Prime Minister David Cameron said: "I'm being kept fully briefed on the major incident in Glasgow. My thoughts are with the families of those involved and the emergency services."
Glasgow City Council leader Gordon Matheson said: "This is a terrible tragedy. Our thoughts and prayers are with all those affected."
A large number of emergency vehicles are in attendance and roads surrounding George Square have been closed.
A Scottish Fire and Rescue Service spokesman said: "Firefighters are in attendance at a serious incident on Queen Street.
"We received a 999 call at 2.30pm reporting a collision and currently have five appliances at the scene.
"Our crews are working alongside other emergency responders to conduct a rescue operation."
A spokeswoman for NHS Greater Glasgow and Clyde said: "Along with all the emergency services, we have an emergency plan that automatically kicks in when there is a major incident."