US actor and filmmaker George Clooney signs autographs during the red carpet event ahead the screening of “Ink” and the Opening ceremony of the 83rd International Venice Film Festival, at Venice Lido

A smiling George Clooney was honoured with a lifetime award at the start of the Venice Film Festival where the long-time Donald Trump critic took another swipe at the US president.

The 65-year-old Oscar winner was called a "legend" by actress friend Laura Dern as she presented him with the gong in front of a star-studded audience at the start of the 83rd edition of the festival.

Clooney, accompanied by his lawyer wife Amal, was cheered by hundreds of well-wishers on the red carpet, while Dern read out tributes from past co-stars including Brad Pitt and Adam Sandler during the ceremony.

"When you start reflecting on the years, you realise you really didn't have a career as much as a series of happy accidents," Clooney told the audience.