Cruise ship collides with a tourist river boat and a dock on a busy Venice ...

This is the terrifying moment passengers topple from a towering cruise ship after it barrelled into a tourist river boat on a busy canal in Venice and injured four female tourists as they tried to run away. 

The collision, which saw the luxury MSC Opera cruise liner crash into the small River Countess and the San Basilio dock, took place at about 8.30 am on the Giudecca Canal- one of the major canals in the ancient city. 

MSC Cruises said the 2,679-passengers onboard the 177-ft high and 902-ft long long liner, which dwarfed the Venice skyline, were approaching the terminal when the ship hit the dock after a technical problem. 

Dramatic footage from the scene shows the moment the cruise ship, apparently unable to halt its momentum and blaring its horns, ploughs into the much smaller river boat as dozens of terrified spectators flee the chaotic scene.   

The collision comes days after seven people were killed and 21 remain missing after the Mermaid, a sightseeing boat, capsized on the River Danube in Budapest, Hungary. 

Medical authorities say four female tourists - an American, a New Zealander and two Australians between the ages of 67 and 72 - were injured falling or trying to run away when the cruise ship rammed into the tourist boat. 

Italian media report that 110 passengers were onboard the small river boat 

In another video, terrified passengers can be heard shouting 'hold on' as the luxury cruise boat crashes into the river boat and then the pier. 

The cruise ship's owner, MSC Cruises, said the ship, the MSC Opera, was about to dock at a passenger terminal when it had a mechanical problem.

The ship then appeared to lose control after a steel cable that tied it to a tugboat snapped.     

Two towboats guiding the cruise ship into Venice tried to stop the massive cruise ship, but they were unable to prevent it from ramming into the small and powerless river boat. 

President of a towboat association in Venice, Davide Calderan, told the Italian news agency ANSA: 'The two towboats tried to stop the giant and then a tow cable broke, cut by the collision with the river boat.'

Mr Calderan said the cruise ship's engine was locked when the captain called for help.

Following the accident, Italian firefighters were deployed to the scene as shocked spectators watched on in horror from the dock. 

Elisabetta Pasqualin was watering plants on her terrace when she heard warning sirens and stepped out to see the crash.

'There was this huge ship in a diagonal position in the Giudecca Canal, with a tugboat near which seemed like it couldn't do anything,' she said.

She described the ship 'advancing slowly but inevitably towards the dock.' 

She said: 'The bow of the ship crashed hard into the bank with its massive weight crushing a big piece of it. Sirens were wailing loudly; it was a very dramatic scene.'

The international cruise ship line with its headquarters in Geneva, says its ship, the MSC Opera, experienced a mechanical problem as it was docking at a passenger terminal and says it is cooperating with authorities to figure out what happened. 

The incident has now reignited calls to ban cruise ships in Venice which have been a source of contention in the over-extended tourist city.

Danilo Toninelli, Italy's transport minister, said 'today's accident in the port of Venice proves that cruise ships shouldn't be allowed to pass down the Giudecca anymore.'

He added: 'After many years of inertia, we are finally close to a solution to protect both the lagoon and tourism.'

The MSC Opera was built in 2004 and can carry over 2,675 passengers in 1,071 cabins. 

According to its sailing schedule, the cruise ship left Venice on May 26 and travelled to Kotor, Montenegro, and Mykonos, Santorini and Corfu in Greece before returning on Sunday to the popular tourist destination of Venice.