Portugal has declared a national day of mourning after an iconic railway cable car crashed in Lisbon, killing at least 16 people and injuring more than 20 others.

The 140-year-old Glória funicular, which is popular with tourists, derailed and crashed into a building near Liberty Avenue at around 18:15 local time (17:15 GMT) on Wednesday.

Foreign nationals are among those killed, authorities said, although the identities of the victims have not been confirmed. Some of the injured are in a serious condition.

Lisbon's mayor Carlos Moedas called it a "tragic day for our city".

Footage shared on social media showed the crumpled yellow funicular overturned in the cobblestone street and people running from the area as smoke filled the air.

Several passengers trapped in the wreckage had to be freed by emergency responders, authorities said.

Officials have not confirmed what caused the crash, but eyewitness reports suggested that the braking system on the cable-hauled funicular failed, sending it hurtling down the steep street and into a building.

A witness told Portuguese TV channel SIC that the funicular, which can carry around 40 people, "hit a building with brutal force and collapsed like a cardboard box; it had no brakes".

It is not known how many passengers were on board when the crash happened.

Portuguese newspaper Observador reported that a cable came loose along the railway's route, causing it to lose control and collide with a nearby building.

One witness told Observador that 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," said Teresa d'Avó.

"But it fell around the bend and crashed into a building."

An investigation has now been launched by Lisbon prosecutors, police and the transport accident investigation department.

The city's public transport operator, Carris, said it had complied with "all maintenance protocols". Interim maintenance was carried out last year and regular monthly and weekly checks had been done, it said.

The company is "very sorry about what happened" and has opened its own investigation.

European Commission President Ursula von der Leyen sent her condolences to the families of the victims, while Portugal's President Marcelo Rebelo de Sousa and Spain's Prime Minister Pedro Sánchez also sent their sympathy.

Sánchez said he was "appalled by the terrible accident".

A funicular is a type of railway system that allows travel up and down steep slopes.

The two cars on the Glória funicular are powered by electric motors. They are attached to opposite ends of a haulage cable meaning that as one travels downhill, its weight lifts the other, allowing them to ascend and descend simultaneously.

The bright yellow Glória funicular is one of the most famous tourist attractions in Lisbon. It was opened in 1885 and electrified three decades later.

The historic vehicles are a crucial part of a city as hilly as Lisbon. They snake their way up many of the cobbled streets.

The one that crashed travels some 275m (900ft) from Restauradores Square in downtown Lisbon, up to Bairro Alto - or high neighbourhood - taking just three minutes to make the journey. (BBC)