The Brit star was honoured for his portrayal of a mourning professor in A Single Man at the September 12 ceremony.
The Bridget Jones's Diary' star gave his speech in Italian.
He told the assembled audience the award was "possibly the greatest honour of my life," reports The Daily Star.
Israeli war film Lebanon, which is shot almost entirely inside an Israeli tank, was handed the prestigious Golden Lion award.
The best director honour went to Iranian-born Shirin Neshat for Women Without Men.
Colin Firth in a scene from A Single Man.
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