On September 28, 1943, two days before Rosh Hashanah, a German diplomat in Denmark, Georg Ferdinand Duckwitz, warned the Danish political leaders (one of whom later became the Danish Prime Minister, as this article indicates) that the Germans were planning to deport all of Danish Jewry to Theresienstadt within days. Duckwitz, a maritime attache, arranged for Sweden to accept the Jewish refugees and delayed the German patrol boats that would have caught the escaping Jews by placing them in dry-dock.
The citizenry of Denmark immediately worked together en masse to arrange for the incredible logistics of transporting thousands of Jews safely to Sweden in only three days. It appears that the final number saved was 7220 Jews and 680 non-Jewish relatives. Of the 500 remaining Jews that were deported, only 51 died, mostly due to Danish pressure on Germany for the well-being of their citizens.
More details can be seen in this article, with additional information here and here.