AlphaTauri driver Pierre Gasly took an improbable first Formula 1 win at Sunday's Italian Grand Prix, in a remarkable race that was turned upside down by a mid-race safety car period and a crash that required the Grand Prix to be halted.

McLaren's Carlos Sainz finished a close second for his best ever finish, and the Racing Point of Lance Stroll took third to complete one of the most unlikely podiums in recent F1 history.

World championship leader Lewis Hamilton could only finish seventh after serving a time penalty for coming into the pitlane when it was closed, dropping him to the back of the field.

Hamilton had controlled the early stages of the race from his pole position, leading away from the McLaren of Sainz and Lando Norris, Racing Point's Sergio Perez, the Renault of Daniel Ricciardo, and Hamilton's Mercedes teammate Valtteri Bottas, who dropped from second to sixth after a poor start.

The race continued in a fairly processional manner until lap 20, when Kevin Magnussen's Haas stopped just before the pit entrance, bringing out the safety car and bunching the pack up.

Mercedes almost immediately elected to pit Hamilton for his scheduled tyre change, but crucially failed to notice that race director Michael Masi had temporarily closed the pit entrance to allow marshals to clear the stricken Haas, earning their driver a 10-second stop-go penalty that put him out of serious contention.

The safety car came in at the end of lap 23, by which time almost all the drivers had made their pit stops legitimately, but almost immediately Charles Leclerc lost control of his Ferrari and crashed heavily at Parabolica, forcing proceedings to be halted while marshals cleared Leclerc's car and repaired damage to the barrier.

The race was restarted from a standing start with Hamilton lining up first ahead of Stroll, who hadn't pitted but had been able to change his tyres during the stoppage, and Gasly, who had made his pitstop just before the safety car was deployed.

After Hamilton peeled off after one racing lap to serve his penalty, Gasly assumed the lead ahead of the Alfa Romeos of Kimi Raikkonen and Antonio Giovinazzi who were followed by Sainz, as Stroll fell down to sixth after a poor start and almost took Raikkonen out as he overshot the Roggia chicane.

By the time Sainz passed Raikkonen for second place on lap 34, Gasly had a four-second lead at the front, and although the Spaniard gradually reeled in the Frenchman, Gasly held firm to take a fairytale maiden Grand Prix win by just 0.4 seconds.

"Honestly, it's unbelievable, I'm not sure I'm realising what's happening right now. It was such a crazy race and we capitalized on the red flag. To get my first win in Formula 1, I am struggling for words," said a disbelieving Gasly after the race.

"This team have given me so much, my first chance, my first podium and my first win. It is just an amazing day."

Gasly's win is the first by a French driver in F1 since 1996, and comes after he was dropped by Red Bull midway through last season and demoted to the then-Toro Rosso team, now called AlphaTauri.

It is also the second F1 victory for the Faenza-based squad, following Sebastian Vettel's equally unlikely win at the 2008 Italian Grand Prix.

Behind the top three, Norris took fourth ahead of an out of sorts Bottas, who looked off the pace all day and appeared to suffer with handling and overheating problems.

Ricciardo finished sixth ahead of Hamilton, whose late-race charge also saw him take the bonus point for fastest lap.

Ricciardo's teammate Esteban Ocon took eighth, with the second AlphaTauri of Daniil Kvyat and Perez rounding out the top ten.

But it was another nightmare weekend for Ferrari on their home turf. Before Leclerc's accident, his teammate Vettel had become the first retirement of the race, pulling off with brake failure on lap six after having qualified a lowly 17th.

It was also a disastrous race for Red Bull, with Max Verstappen retiring from the midfield with engine trouble, and Alex Albon finishing down in 15th after a scrappy performance in which he received a five-second penalty for impeding Romain Grosjean's Haas.

Despite a rare disappointing day, Hamilton still convincingly leads the drivers' championship with 164 points. Second-placed Bottas closes the gap slightly with 117 points, while Verstappen remains on 110 points and drops to third.

In the constructors' standings, Mercedes stay out in front with 281 points. Red Bull remain second with 158 points, while third-placed McLaren now have 98 points after their best result of the year.

The next round of the 2020 season sees Formula 1 remain in Italy and head to Mugello for the first time, for the inaugural Tuscan Grand Prix.