This afternoon in Trafalgar Square: an amazing celebration of cinema and diversity

'Being a Londoner is magic!' So said filmmaker Mike Leigh this afternoon to thousands in Trafalgar Square, and despite the cold, damp February weather no one there would've argued with him.

The crowd was gathered, amazingly, for a film premiere. 'The Salesman' is Iranian writer-director Asghar Farhadi's latest work, and later tonight it may well pick up the Best Foreign Language Film award at the Oscars. But Farhadi won't be there to collect it: first he was shut out by Donald Trump's travel ban, then when the ban was temporarily lifted by a judicial order he opted not to come anyway, as a protest. His decision inspired Leigh, actress Lily Cole and other notables to plan a screening of the film in Grosvenor Square opposite the American Embassy. But when permission wasn't granted, they teamed up with Mayor of London Sadiq Khan to present the film, for free, in Trafalgar Square.

The atmosphere before the screening was electric. London's Iranian population turned out in force, but they were by no means alone. As Khan said in his rousing introductory speech, 'Whether you're from Iran or Iraq, Streatham or Shoreditch, Lebanon or London, you are welcome.' Scheduled as part of the ongoing 'London is Open' project, the screening was meant to remind us all that London has always been a city of immigrants, and that we can all benefit from the culture newcomers bring, be it film, food or music. 'The thing about this city,' Khan went on, 'is that we don't just tolerate difference. We respect it, we celebrate it and we embrace it. Those are the foundations on which the success of our city was built.'

The focus of the day was clearly on film. In an enthused and gracious speech, Mike Leigh talked about his love of shooting in the capital ('it's always buzzing with activity') and gave a big shout out to the BFI and the London Film Festival. But politics kept sneaking back in: Leigh called Trump's travel ban a 'cynical, divisive and disgusting policy', while compere Mariella Frostrup described it as 'a development worthy of a Kafka novel'. Not all political statements were officially welcome. A group of Iranian refugees holding up banners decrying the country's repressive policies were asked to put them away by Trafalgar Square stewards. They promptly refused, to cheers from nearby punters.

'The Salesman' wasn't, perhaps, the ideal choice for a packed outdoor event – it's terrific film, but also decidedly chatty. Combined with the fact that a light drizzle kicked up the second it started, this meant that the square might not have been quite as rammed by the final credits as it had been at the beginning. But with a closing performance by the Orchestra of Syrian Musicians – accompanied by noted world music enthusiast Damon Albarn – it was worth sticking around to the end. Sadiq clearly thought so, too.

We Londoners love to grumble about our city. But then an event like this comes along and you remember, as Khan bellowed proudly this afternoon, that you really are living in 'the greatest city in the world'. Cheers Sadiq. Same time next year?