Just days before US President Donald Trump and other leaders arrive in Sicily for the G7 summit, an alleged mafia don was shot dead in broad daylight on the gritty streets of Palermo.

Giuseppe Dainotti, regarded as a senior godfather in Sicily's Cosa Nostra mafia, died today after two men riding a motorbike drew up alongside him and shot him in the head, the Daily Telegraph reports.

The killing bore all the hallmarks of a professional mafia hit. Police believe he was killed by rivals within Cosa Nostra, possibly over a drug turf war, amid reports that he had been a "dead man walking" for at least a year.

It was the first killing of an alleged mafia boss in Sicily for three years. The 67-year-old was riding a bicycle when he was shot dead.

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"I heard two gun shots. It was about 7.50am," said a local woman. "I thought they were fireworks at first - people round here let them off at all times. I looked outside and I saw a man lying on the ground, losing blood from his head. There was no one else around. As soon as I realised that a murder had been committed I was terrified."

Dainotti had served a lengthy prison term for murder, drug trafficking and robbery but was released from jail in 2014.

"When people say the mafia is finished or has been defeated, sooner or later something happens that shows that the mafia is still here," said Francesco Lo Voi, a prosecutor in Palermo.

Trump and the leaders of Britain, France, Germany, Italy, Japan and Canada will travel to the picturesque resort town of Taormina on Friday night NZT for the two-day Group of 7 summit.

Taormina is at the opposite end of Sicily from Palermo and there is almost zero risk of there being any risk to the leaders and their delegations. The town, which sits on a cliff overlooking the sea, will be in lockdown and protected by around 10,000 soldiers and police.

But the execution of a mafia kingpin on a public street is hardly the image that Sicily wants to project to the world as it prepares to host the high-profile summit, which Trump is attending as part of his first international trip.