ROME — A colossal cruise liner plowed into a smaller tour ship and a wharf on a canal in Venice on Sunday morning, injuring four people and reigniting arguments about the dangers of allowing the huge vessels to pass through the fragile lagoon city.

Footage of the crash showed the cruise liner, the approximately 900-foot-long MSC Opera, blaring its horn as it hit the wharf and crashed into the tour ship, the River Countess, which was docked at the San Basilio Terminal on the Giudecca Canal, where passengers often disembark from smaller vessels.

The accident occurred around 8:30 a.m. Videos taken from the dock showed the ship heading straight for the wharf, unable to stop, while people on the quay ran away in panic. Four people from the cruise ship were treated for light injuries, the Italian news agency ANSA reported.

The MSC Opera was approaching the cruise ship terminal in Venice to dock when it had a “technical problem,” the ship’s operator, MSC, said in a statement. The company said that the ship had been accompanied by two tugboats when it hit the wharf and the smaller boat at San Basilio.