It was just after 18:00 on Wednesday when a carriage on Lisbon’s famous Gloria funicular careened around the bend of a steep cobblestoned street, crashed into a building, and crumpled, eyewitnesses said.

The carriage “lost control”, descending at full speed and crashing on its side, Helen Chow, who was at the base of the hill, told the BBC.

It sounded like a bomb, she said, followed by “complete scary silence…There was pitch black smoke. Once it dissipated, you saw exactly what happened.”

People were frantic and crying, with others running to help, she described.

“It was awful,” she said. “I am shaken.”

Police are still investigating the cause of the crash, which killed at least 17 people and injured 20 more, some critically, near Lisbon’s Avenida da Liberdade in the Portuguese capital.

Video verified by the BBC shows the crashed yellow-and-white train against the building on the bend of a hill, with another train stopped at the bottom. People are running up the incline towards the scene of the crash.

The two carriages of the 140-year-old funicular, which runs on electric motors, are attached to a cable that enables one to travel downhill while the other goes uphill, passing each other briefly along the three-minute one-way journey.

Witnesses described how the carriage near the bottom of the hill, which was starting to ascend, crashed a short distance backwards just before the upper carriage raced down the incline and into the building.

Around a minute before the crash, Ms Chow, who is originally from Canada but was visiting Lisbon, said she heard a loud screech.

She saw the bottom carriage fall back, past the white line where it usually halted, and make “a hard stop” at the end of the tracks.

A passenger on that carriage told Portuguese news outlet Sic Notícias it only went up a few metres before a “big bang” and “black smoke”.

“People fell to the ground and everyone was screaming inside,” they said.

Ms Chow also saw black debris and heard screaming, as the driver rushed to open the gates to the entrance.

“People jumped out of the window of that tram,” she said. “Just as this happened, I saw the incident tram crash over into the building next to the Subway restaurant.”

Another witness said they ran to help after the lower carriage dropped – before seeing the other coming down “out of control”.

“We only had time to escape, turn our backs and run,” they said. “It came down and struck the building at high speed.”

Teresa d’Avó said the carriage “hit a building with brutal force and collapsed like a cardboard box,” telling television channel SIC it seemed like it “had no brakes”.

She also told newspaper Observador the vehicle was “out of control, without brakes”.

“We all started running away because we thought [the carriage] was going to hit the one below,” she said. “But it fell around the bend and crashed into a building.”

Farid Shovro, owner of a nearby store who ran to the scene, told Observador: “I’ve never seen so many people dead together.”

Some tourists told the BBC they had almost taken the funicular at the time of the crash.

Eric Packer, from the US but visiting Lisbon on holiday, told the BBC he had discussed with his friends taking the cable car and took pictures at 18:00 and 18:01, but decided to walk back to their hotel instead.

They walked about 60 metres and heard a loud crash noise “like a rock falling, like a dump truck had dropped a load of rocks” at 18:02.

They turned around to see dust coming out of the alley about 45 metres behind them and walked back to see what happened. At first, he thought it was the train at the bottom that fell, until he turned and saw the other train that was above it, and realised “the magnitude of what had taken place”.

His photograph shows the yellow-and-white train, a tangle of metal, on the corner of the narrow alley under a Subway restaurant sign, with the other train at the bottom of the hill below it.

“People (were) walking up and running up to try and help,” he said. “Horrible tragedy and our thoughts and prayers go out to the families and survivors.”

Additional reporting by Alex Akhurst, Bernadette McCague and Marina Costa