The two remaining towers of the motorway bridge in the Italian city Genoa that collapsed last year killing 43 people were demolished on Friday to make way for a new structure, authorities said.

The remains of the 4,500-tonne Morandi bridge were detonated at 9:37 am (0737 GMT) and took just seven seconds to collapse in a cloud of dust, city authorities said.

A general view shows the two pylons of Genoa's Morandi motorway bridge seen in red on the structure below, on June 27, 2019 in Genoa. Some of the remains of Genoa's Morandi motorway bridge were destroyed on June 28, 2019, almost eleven months after its partial collapse during a storm killed 43 people and injured dozens. [Photo: AFP/Vincenzo Pinto]

Nearly 4,000 residents had been evacuated ahead of the demolition, and water tanks were placed around the towers to prevent the spread of dust.

Some 400 security forces personnel were also deployed to the area.

Cannons began shooting high towers of water into the air below the bridge seconds before the explosion, which ripped the bridge in two before bringing the giant structure crashing down.

A general view shows the two pylons of Genoa's Morandi motorway bridge seen in red on the structure below, on June 27, 2019 in Genoa. Some of the remains of Genoa's Morandi motorway bridge were destroyed on June 28, 2019, almost eleven months after its partial collapse during a storm killed 43 people and injured dozens. [Photo: AFP/Vincenzo Pinto]

A deafening boom resounded across the north Italian city milliseconds after the collapse as alarms rang out.

The bridge collapsed in August during a heavy rainstorm, sending about 35 cars and several trucks plunging 45 metres (150 feet) onto railway tracks below.

- 'Super heavy' -

Locals and officials including Deputy Prime Ministers Matteo Salvini and Luigi Di Maio witnessed the blast.

The pair had spent several days at the site when it collapsed in bad weather on August 14, 2018, expressing their indignation and arguing over what action to take in the aftermath of the incident.

Italy’s Prime Minister Giuseppe Conte (C) walks by during a visit to the demolition site of the western section of the remains of the Morandi Bridge in Genoa on February 8, 2019, to launch the official start of demolition of the first meters of the bridge's western section. [Photo: AFP/Marco Bertorello]

The disaster threw the spotlight on Italy's creaking infrastructure -- far-right leader Salvini suggested EU budget spending demands from Brussels had limited Italy's ability to maintain it.

The dismantling of the bridge's remains started in February.

Ahead of the demolition, residents expressed relief that the eyesore would finally be gone, but were also concerned about the possible consequences of the detonation.

The bridge "is high, it is big, it is full of iron, it is super heavy... I don't know what will happen when it falls. Apart from the dust it will cause, I am worried about the impact," local man Francesco Russo told AFP TV on Thursday.

With the demolition over, officials are now waiting for the arrival of a ship transporting the first parts of a new bridge, the construction of which is scheduled to begin next year.

The government has promised that the new steel and concrete motorway bridge, designed by Italian architect Renzo Piano, will be open for traffic in April 2020.

- 'Pay dearly' -

Meanwhile, controversy persists over concessions enjoyed by Italian infrastructure group Atlantia, whose majority-owned subsidiary Autostrade was responsible for maintenance works on the bridge.

Di Maio, who is also economic development minister, insisted Wednesday that Atlantia would have concessions revoked in a move that would cost it -- and its main shareholder, the Benetton family -- dearly.

The group would therefore be unable to take part in the rescue of troubled national airline Alitalia, he said.

A reinforced concrete structure is pictured at the site of the Genoa's Morandi motorway bridge on June 27, 2019 in Genoa. [Photo: AFP/Vincenzo Pinto]

His comments sparked a drop in Atlantia shares and the company threatened legal action.

Business leaders and politicians on both left and right accused Di Maio of triggering market speculation.

Salvini said Atlantia employs thousands of people and warned against "summary judgements".

Atlantia, a world leader in motorway and airport concessions, employs some 30,000 people worldwide.

Italian Prime Minister Giuseppe Conte has said the revocation procedure is ongoing.

"The serious breach (of responsibility) is an objective fact, we cannot pretend that nothing has happened," he said.